revset: reword commonancestor()'s help

The new version seems a bit more consistent with other doc comments,
and feels clearer to me (doesn't explain "commonancestors(set)" as
"common ancestors of set").

Differential Revision: https://phab.mercurial-scm.org/D4740

mercurial/revset.py

@@ -623,7 +623,7 b' def _commonancestorheads(repo, subset, x'
 
 @predicate('commonancestors(set)', safe=True)
 def commonancestors(repo, subset, x):
-    """Returns all common ancestors of the set.
+    """Changesets that are ancestors of every changeset in set.
     """
     startrevs = getset(repo, fullreposet(repo), x, order=anyorder)
     if not startrevs: