How to document Oracle database?
In Oracle, you would use the COMMENT command to:


  [...] add to the data dictionary a comment about a table or table
  column, view, materialized view, operator, indextype, mining model, or
  edition.


Most tools (PL/SQL Developer, Toad...) will display these comments in appropriate fields when you browse the database schema.

The comments can be queried directly with the *COMMENTS dictionary views, such as ALL_TAB_COMMENTS.