Documents CHANNEL(musicclass) taking priority over m([x]) in waitExten

Documents CHANNEL(musicclass) taking priority over m([x]) in waitExten

If waitExten specifies a music class to use with its music on hold option, it will use
CHANNEL(musicclass) instead if that channel variable has been set on the initiating
channel.  This documents that behavior in the waitExten app so that this can be known
without checking the documentation of the code in function local_ast_moh_start.

(closes issue ASTERISK-18804)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@347239 65c4cc65-6c06-0410-ace0-fbb531ad65f3