Fix options 'm' and 's'. They were swapped in the code. Also document the...

Fix options 'm' and 's'. They were swapped in the code.  Also document the fact that app_confbridge does not automatically answer the channel.

(closes issue #15964)
Reported by: shrift


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@220904 65c4cc65-6c06-0410-ace0-fbb531ad65f3