Correct documentation for ConfbridgeList AMI action

The documentation for ConfbridgeList states that the Conference field is
optional. That's not really the case: if you fail to provide a Conference
number, the command will kick back an error.

(closes issue AST-1090)
Reported by: John Bigelow
........

Merged revisions 380028 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@380029 65c4cc65-6c06-0410-ace0-fbb531ad65f3