Update documentation for MESSAGE_SEND_STATUS variable.

(Closes issue ASTERISK-19056)
Reported by: Yuri
Patches:
     348360.diff uploaded by Yuri (license #5242)
........

Merged revisions 348605 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348606 65c4cc65-6c06-0410-ace0-fbb531ad65f3