Document the RemoveUnusedDeclarations pass. (#548)
Document the RemoveUnusedDeclarations pass. (#548)

* Add Doxygen-style comments for the RemoveUnusedDeclarations pass.  Also make giveWarning() private, since it is only used inside the pass.

* Clarify the purpose of the warning list (and remove an inaccurate TODO comment).  Based on feedback from @mbudiu-vmw and @ChrisDodd.

mbudiu-vmw/p4c-clone,p4lang/p4c,hanw/p4c,hanw/p4c,hanw/p4c,mbudiu-vmw/p4c-clone,p4lang/p4c,mbudiu-vmw/p4c-clone,mbudiu-vmw/p4c-clone,p4lang/p4c,p4lang/p4c,hanw/p4c,p4lang/p4c