Tell users how to see the complete list of commands

Using "repo help --all" may not be obvious.

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/help.py

@@ -77,6 +77,7 @@ The most commonly used repo commands are:
       print fmt % (name, summary)
     print """
 See 'repo help <command>' for more information on a specific command.
+See 'repo help --all' for a complete list of recognized commands.
 """
 
   def _PrintCommandHelp(self, cmd):