match: document bad callback semantics

mercurial/match.py

@@ -81,6 +81,13 b' class match(object):'
         for f in self._files:
             yield f
     def bad(self, f, msg):
+        '''callback for each explicit file that can't be
+        found/accessed, with an error message
+
+        return True to keep file in results, False to discard
+
+        default behavior is to silently keep
+        '''
         return True
     def dir(self, f):
         pass