Clean up documentation

This patch cleans up documentation in func_channel for the following items:
* rtpsource
* secure_signaling
* secure_media
* various OOH323 parameters

(closes issue ASTERISK-20969)
Reported by: snuffy
patches:
  func_chan-update.diff uploaded by snuffy (License 5024)
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Merged revisions 394980 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 394981 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394982 65c4cc65-6c06-0410-ace0-fbb531ad65f3