revset: remove part of the commonancestors() comment

The reason is that:
- it shows up in "hg help revset", but it doesn't look like
  documentation targeted at users
- it doesn't make sense to me: it doesn't say what happens with < 2
  revisions, and is not quite right because my understanding is that
  this revset was created precisely because "::x and ::y" was not
  quite the same (when x and y don't evaluate to singletons).

Differential Revision: https://phab.mercurial-scm.org/D4739

mercurial/revset.py

@@ -624,12 +624,6 b' def _commonancestorheads(repo, subset, x'
 @predicate('commonancestors(set)', safe=True)
 def commonancestors(repo, subset, x):
     """Returns all common ancestors of the set.
-
-    This method is for calculating "::x and ::y" (i.e. all the ancestors that
-    are common to both x and y) in an easy and optimized way. We can't quite
-    use "::head()" because that revset returns "::x + ::y + ..." for each head
-    in the repo (whereas we want "::x *and* ::y").
-
     """
     startrevs = getset(repo, fullreposet(repo), x, order=anyorder)
     if not startrevs: