Update MixMonitor documentation.

Updated the MixMonitor documentation for the 'b' option so that
it is more obvious that you must not optimize awat the Local
channel when using this option.

(issue #14829)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@197895 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_mixmonitor.c

@@ -70,6 +70,10 @@ static const char *desc = ""
 " b      - Only save audio to the file while the channel is bridged.\n"
 "          Note: Does not include conferences or sounds played to each bridged\n"
 "                party.\n"
+"          Note: If you utilize this option inside a Local channel, you must\n"
+"                 make sure the Local channel is not optimized away. To do this,\n"
+"                 be sure to call your Local channel with the '/n' option.\n"
+"                 For example: Dial(Local/start@mycontext/n)\n"
 " v(<x>) - Adjust the heard volume by a factor of <x> (range -4 to 4)\n"	
 " V(<x>) - Adjust the spoken volume by a factor of <x> (range -4 to 4)\n"	
 " W(<x>) - Adjust the both heard and spoken volumes by a factor of <x>\n"