In Revit you can place symbols within text, like diameter, degree etc.
But the symbols you can place in text are limited to those available within the font you are using.
Arial is the recommended font as pretty much every computer and printer has it.
There are two methods to place symbols:
WINDOWS CHARACTER MAP
Start the Character Map program.
Under Windows Start (bottom left corner of desktop) > All Programs > Accessories folder > System Tools folder
(or type Map in Search programs and files)
Leave it open, when you need to place a symbol make it active and select your symbol, hit Select button, then Copy button.
Then in Revit paste into the text you are writing in (Ctrl C).
Note that Character Map also shows you the keyboard shortcut for each character bottom right:
KEYBOARD SHORTCUTS
You can insert a symbol whilst in the text command by holding the Alt key and typing the ASCII number of that symbol.
But it will only work if you use the NUMPAD, not numbers at the top of your keyboard.
The following are a few useful ALT-key combinations from Arial font:
° = ALT + 0176
± = ALT + 0177
² = ALT + 0178
³ = ALT + 0179
× = ALT + 0215
Ø = ALT + 0216
÷ = ALT + 0247