Address Clang warnings are overloaded virtual functions in UI_GADET. Also opportunity to simplify code by making the most of inherited polymorphism. Docs cleanup

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10861 387891d4-d844-0410-90c0-e4c51a9137d3
Quieten a number of warnings related to UI_USERBOX without a declaration yet. Added TODO.

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10862 387891d4-d844-0410-90c0-e4c51a9137d3