Update MeetMe p and X option documentation when interacting with the s option.

Update MeetMe p and X option documentation when interacting with the s option.

ASTERISK-12175 changed the p and X options to not interfere with the s
option when they are used together.  It makes more sense for the s option
to have priority for the DTMF '*' key since it cannot change its
activation code.  Otherwise, you could not use option s with the p or X
options.

JIRA AST-671
........

Merged revisions 340470 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@340471 65c4cc65-6c06-0410-ace0-fbb531ad65f3