annotate: describe --skip as taking a revset

It's obvious to me, but probably wouldn't be obvious to a novice user.

Differential Revision: https://phab.mercurial-scm.org/D7557

mercurial/commands.py

@@ -362,7 +362,7 b' def addremove(ui, repo, *pats, **opts):'
             b'',
             b'skip',
             [],
-            _(b'revision to not display (EXPERIMENTAL)'),
+            _(b'revset to not display (EXPERIMENTAL)'),
             _(b'REV'),
         ),
     ]