Confbridge: Add references for kick all option

After the ability to kick all attendees from a conference was added, a
rework removed the comment about that feature from the CLI
documentation. This adds that documentation and adds "all" to the
participant tab completion list for the confbridge kick command.

(closes issue ASTERISK-23282)
Reported by: Dorian Logan
........

Merged revisions 412728 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@412729 65c4cc65-6c06-0410-ace0-fbb531ad65f3