Move music on hold related applications documentation to XML.

Move MusicOnHold, SetMusicOnHold, StartMusicOnHold, StopMusicOnHold static
documentation to the new AstXML form.

(issue #15245)
Reported by: eliel
Patches:
      res_musiconhold_static_conversion.txt uploaded by lmadsen (license 10)
      (with some fixes and formatting by me)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@199413 65c4cc65-6c06-0410-ace0-fbb531ad65f3