match: remove doc about undefined behavior of visitdir()

This was added by 8545bd381504, but core matchers support visitdir() of
arbitrary locations since 2773540c3650, and verifier._verifymanifest()
doesn't seem to strictly obey the restriction.

I have no idea how important this API contract is for third-party extensions.
That's why this patch is RFC.

mercurial/match.py

@@ -305,9 +305,6 b' class basematcher(object):'
         Returns the string 'all' if the given directory and all subdirectories
         should be visited. Otherwise returns True or False indicating whether
         the given directory should be visited.
-
-        This function's behavior is undefined if it has returned False for
-        one of the dir's parent directories.
         '''
         return True