object.c: [DOC] merge rdoc

* gc.c (rb_obj_id): remove unused rdoc.

* object.c (rb_obj_hash): [DOC] merge unused rdoc from rb_obj_id()
  in gc.c.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45338 b2dd03c8-39d4-4d8f-98ff-823fe69b080e