Add documentation to function CHANNEL for options echocan_mode and buffers

Add documentation to function CHANNEL for options echocan_mode and buffers

The ability to set "echocan_mode" and "buffers" through the dialplan was added
to chan_dahdi some time ago.  This patch adds some documentation to
func_channel.

(Closes issue ASTERISK-19911)
Reported by: Dale Noll
Tested by: Michael L. Young
Patches:
  asterisk-19911-branch18.diff uploaded by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/1949/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@368092 65c4cc65-6c06-0410-ace0-fbb531ad65f3