match: add comments to explain explicitdir and traversedir

mercurial/match.py

@@ -119,7 +119,11 b' class match(object):'
         found/accessed, with an error message
         '''
         pass
+    # If this is set, it will be called when an explicitly listed directory is
+    # visited.
     explicitdir = None
+    # If this is set, it will be called when a directory discovered by recursive
+    # traversal is visited.
     traversedir = None
     def missing(self, f):
         pass