ConfBridge: Fix channel parameter documentation

ConfBridge: Fix channel parameter documentation

Confbridge AMI and CLI commands for mute, unmute, and setting the
single video source can accept channel prefixes in lieu of a full
channel name, but documentation states only that it is required and is
a channel name. This corrects the documentation.

(closes issue PQ-1397)
Reported by: Steve Pitts
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Merged revisions 406217 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 406223 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@406224 65c4cc65-6c06-0410-ace0-fbb531ad65f3