audiod: Document and fix command handler return values.

It has always been the case that a positive return value from a
command handler causes audiod to dump all audiod status items to all
clients. This behaviour is not documented though, and some command
handlers get it wrong. In fact, the help, stat, tasks, grab, version
commands all return positive values although those commands never
change any status items.

This comment documents the meaning of the return value and fixes up
the above command handlers.