localrepo: add a note about parentenvvar

Review feedback from Pierre-Yves David.

mercurial/localrepo.py

@@ -1214,6 +1214,8 b' class localrepository(object):'
     def _lock(self, vfs, lockname, wait, releasefn, acquirefn, desc,
               parentenvvar=None):
         parentlock = None
+        # the contents of parentenvvar are used by the underlying lock to
+        # determine whether it can be inherited
         if parentenvvar is not None:
             parentlock = os.environ.get(parentenvvar)
         try: