Add docstring to cmd.Cmd.do_help()
diff --git a/Lib/cmd.py b/Lib/cmd.py
index 85282f8..8fa7d01 100644
--- a/Lib/cmd.py
+++ b/Lib/cmd.py
@@ -288,6 +288,7 @@
         return list(commands | topics)
 
     def do_help(self, arg):
+        'List available commands with "help" or detailed help with "help cmd".'
         if arg:
             # XXX check arg syntax
             try: