Clean up documentation

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

(closes issue ASTERISK-20969)
Reported by: snuffy
patches:
  func_chan-update.diff uploaded by snuffy (License 5024)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@394980 f38db490-d61c-443f-a65b-d21fe96a405b