revlog: update the docstring of `ancestors` to match reality

Code using this method expect the revision to be (reverse) sorted. As pointed by
Yuya Nishihara, the docstring should reflect that.

mercurial/revlog.py

@@ -768,7 +768,7 b' class revlog(object):'
         return chain, stopped
 
     def ancestors(self, revs, stoprev=0, inclusive=False):
-        """Generate the ancestors of 'revs' in reverse topological order.
+        """Generate the ancestors of 'revs' in reverse revision order.
         Does not generate revs lower than stoprev.
 
         See the documentation for ancestor.lazyancestors for more details."""