Update documentation to make it explicit that "stream file" will not restart musiconhold.

(issue ASTERISK-17367)
Reported by: oej
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Merged revisions 373989 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373990 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373991 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373992 65c4cc65-6c06-0410-ace0-fbb531ad65f3