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.
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Merged revisions 379310 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 379311 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@379312 65c4cc65-6c06-0410-ace0-fbb531ad65f3