I'm not sure it makes sense to document the undesirable/undefined behavior here. People should know that they must use this command while connected to a mongos, and otherwise the helper won't do what they want. full stop. We don't want people relying on this implementation detail (i.e. that it's a wrapper around an insert) or becoming disgruntled over a few bytes of data. 

If in a later version, we change the implementation of sh.addshard to use the dbcommand, you'll have to use it against the mongos anyway and we save our selves from having to update this documentation. 

Furthermore, we are careful to recommend throughout the manual that the only interaction with a sharded cluster should be through the mongos (except in exceptional situations,) so this little bit is basically saying "if you ignore what we suggest elsewhere, something annoying might happen.

In this direction, In addition to this edit, I think this should be either an `important` or a `note` directive...