branchcache: fix the docstring of _verifybranch()

Initially the function was designed to support verifying all branches but later
I decided to have a separate function. I forget to remove the doc related to
that. Thanks to Yuya for spotting this in review.

Differential Revision: https://phab.mercurial-scm.org/D6239

mercurial/branchmap.py

@@ -189,8 +189,7 b' class branchcache(object):'
         self._closedverified = True
 
     def _verifybranch(self, branch):
-        """ verify head nodes for the given branch. If branch is None, verify
-        for all the branches """
+        """ verify head nodes for the given branch. """
         if branch not in self._entries or branch in self._verifiedbranches:
             return
         for n in self._entries[branch]: