SQL Developer Data Modeler allows to generate database documentation and Entity-Relationship diagrams.
Get 'SQL Developer Data Modeler' from http://www.oracle.com/technetwork/developer-tools/datamodeler/downloads/index.html and install it.
Reference > https://dataedo.com/blog/how-to-generate-database-documentation-with-oracle-sql-developer-data-modeler#describingtables