Mention ipa COMMAND --help as the preferred way to get command help

This avoids the problem with ambiguous command/topic names.

No functionality is changed; `ipa help <COMMAND>` still works as before
if there's no topic with the same name.

https://fedorahosted.org/freeipa/ticket/3247

ipalib/cli.py

@@ -664,7 +664,10 @@ class help(frontend.Local):
     Display help for a command or topic.
     """
 
-    takes_args = (Str('command?'),)
+    takes_args = (
+        Str('command?', cli_name='topic', label=_('Topic or Command'),
+            doc=_('The topic or command name.')),
+    )
     takes_options = (
         Any('outfile?', flags=['no_option']),
     )