I'm not an expert in this field, as anyone who's ever read my work would know, but I've got some advice for people who are writing for games, both video and tabletop, though primarily tabletop games.
Consider both the system and narrative impacts of a description when putting it into the game, whether as a tooltip in a video game or a blurb about something in a tabletop game. Don't include references to something there's no system support for.
For instance, I saw a description of speed on a tooltip in a game which stated that "Speed makes attacks more frequent.". In terms of narrative, this is totally true and makes sense, but the system didn't give another attack regardless of how much speed a person had; they just got to move first. So a description like "Characters with a high speed land their attacks first." would resolve this confusion by providing a narrative. However, the game it came from had a JRPG-styled combat system, so it would be inaccurate to state "Characters with a high speed move faster.", since there is no movement to be had in the game's system, though it would work fine in the narrative sense.
Be sure to close up prepositional phrases and generally make sure a sentence is complete with appropriate nouns and pronouns; assuming a player will look at something a certain way can be a major mistake. This is especially important on the tabletop, where the system's play comes directly from the text.
A quick example would be "Boosting attack improves damage.". This is not, in and of itself, wrong. However, it would be better to write the description as "An increased attack score raises a character's damage output." or a similar appropriate phrase, just because the previous one leaves a couple things undefined; "improves" does not necessarily refer to a larger amount; "damage" can refer to incoming or outgoing, though this should be clearly outgoing by context; boosting is almost always associated with a positive change but can be rather vague. It can also help to include the added noun for "character" just to provide the implied noun and make the sentence entirely unambiguous.
Include an example. This can be difficult for a tooltip in a video game (though a UI designer could probably figure out some neat ways to put in a sub-tooltip containing an example without making the normal one bloated and take up half a screen), but there is no excuse for not including one in a tabletop game. This doesn't necessarily have to be highlighted in bold text screaming "EXAMPLE! EXAMPLE! EXAMPLE!" but you can include them stealthily; remember that when writing a tabletop game everything should serve both as narrative and system mechanic.
The following is an excerpt from the section on religion and cosmology in my upcoming game Adventures in Aduelle:
are chosen mortal servants of Jutil who are given astronomical
amounts of power, with the caveat of Jutil altering their
consciousness to prevent self-interest from interfering with their duties. The exact power and form of each Ascendant varies,
though most Ascendants are capable of conjuring a form of their
mortal selves and all are immortal. Ascendants are capable, unlike
Jutil, of leaving and entering the world of Aduelle at will, either
residing with Jutil in a realm outside the known universe or
returning to a different point within Aduelle. Individual Ascendants
cannot be destroyed by Jutil without the destruction of all
Ascendants, a rare example of something that seems to cause issues for
Jutil's plans for Aduelle.
In doing this, I tried to do several things; Aduelle's mechanics are still unfinished, and Ascendants are powerful enough they don't really fit within the mechanical system, so there's no rules for actually encountering an Ascendant face-to-face like there will probably be in the finished version.
First, I stress the power of Jutil; if he can create powerful beings he must himself be powerful or have a way of reaching great power, which is important because Jutil's basically the monotheistic deity (but not sole powerful cosmological figure) of Aduelle. I also make it so that Ascendants lack a certain degree of free-will and have a clear purpose outside of rooting on certain mortals or others, meaning that players cannot patronize one Ascendant and gain access to a powerful ally without adhering to Jutil's codes. There are exceptions to this rule, but this also means that a player cannot become an Ascendant within the rules of Adventures in Aduelle (or if they can, it carries restrictions normal adventurers would lack). I point out that Ascendants maintain their individuality as well, make them capable of miraculous abilities of teleportation and true immortality not possible by magic, make them relatively stable, and create a difficulty within the game that explains why rogue Ascendants are not necessarily destroyed as soon as they rebel. However, the first paragraph is not an example, it's a declaration; and it is vague for a number of reason.
Following that, I give an example of a historical figure who is now an Ascendant; Azet was the founder of Azekal and the Eternal Empire way back after the Shattering (which is also touched on, but takes place several hundred years before the setting of Adventures in Aduelle, which is itself a couple decades after Constructs of Azazael). I give a brief history for those who haven't read the biography of Azet to explain some of his achievements, and the reasons for his motivations; then I give his typical behavior-he helps his followers when they contact him, and ensures that the nations he established remain in the state he left them (no pun intended). I then give some information to tell anyone running the game how an encounter with Azet may play out; and end it by pointing out that he is not a rogue Ascendant and he can intervene in the affairs of supplicants to Jutil.
The reasons for this are simple; not only does it hopefully serve to create a more rich and engaging world for the reader to enjoy, but it also provides guidelines for other Ascendants, who basically form a pantheon similar to the gods of Greek mythology.
So as you can see, my attempt at giving an example was pretty subtle as far as examples go, but it's the narrative equivalent of providing equations and running through simulated dice rolls in video games or tabletop games, and it's the sort of thing that is crucially important to me explaining what's going on there.
However, don't get too cocky when writing a mechanical example; they're a lot more simple but there's still a lot of pitfalls to avoid.
First, be sure you cover extreme situations as well as the normal ones; if there's some special rule in play occasionally that could be vague, cover it. Personally, I think it's good to have an example for every rule and possible success/failure outcome.
For instance, if your game includes a rule for grazing blows that barely succeeded, and your section on doing damage just contains the following example, you could have a very real problem.
Example: Mark sings his sword at Luke, rolling a 11. The difficulty to hit Luke is 10, so Luke gets a nasty gash from Mark's swing, taking such and so much damage.
There's nothing wrong with this example, but if it's the only way we get an insight into combat, we're going to question how it works. There's two ways to do this; you can add another example, or amend your example with a new rule.
Example: Luke retaliates at Mark with a dagger, hoping to meet the difficulty of 8 to hit Mark. Luke's dice come up as exactly 8, so Luke hits, but only does half of such and so much damage to Mark.
The additional example works if the rule is written far from the original rule or if it's an optional rule, but I'd recommend attaching a quick follow-up to the original example. This is better more because while if there's a a ton of examples the game will make a lot more sense, but if every other line is an example it is more tempting to assume you understand the rules and skip them; merging the two examples into one can be a little less clear, but unless you've got a good running narrative with the examples (Degenesis did this well, its examples almost formed a pseudo-narrative, and I've seen games that do basically a normal play session as an example, which works well also) they tend to be skipped after the fifteenth or so example in a section.
Example: Mark sings his sword at Luke, rolling a 11. The difficulty to hit Luke is 10, so Luke gets a nasty gash from Mark's swing, taking such and so much damage. If Mark's roll had been 10, he would have only done half of that damage to Luke, resulting in half of such and so much damage.
Note that this rule includes halving, so I'd make the damage (if possible) be a number that shows off whatever system is method to round the number in that case.
Note that these sorts of examples probably wouldn't be included within a video game, except in a tutorial, manual, or help section, but the general rules apply. In addition, it is important to make sure that the rules are clearly stated as well as the examples; being good at writing one is not an excuse for omitting another.