Further fix misinformation in the description of manager MailboxStatus command.

The description still claimed that it returned the number of messages rather than
whether there were messages waiting.
........

Merged revisions 379310 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@379311 65c4cc65-6c06-0410-ace0-fbb531ad65f3