Update documentation in MixMonitor.

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

(closes issue #14829)
Reported by: licedey
Tested by: mmichelson, licedey, lmadsen

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@197828 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_mixmonitor.c

@@ -68,6 +68,9 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 					<option name="b">
 						<para>Only save audio to the file while the channel is bridged.</para>
 						<note><para>Does not include conferences or sounds played to each bridged party</para></note>
+						<note><para>If you utilize this option inside a Local channel, you must make sure the Local
+						channel is not optimized away. To do this, be sure to call your Local channel with the
+						<literal>/n</literal> option. For example: Dial(Local/start@mycontext/n)</para></note>
 					</option>
 					<option name="v">
 						<para>Adjust the <emphasis>heard</emphasis> volume by a factor of <replaceable>x</replaceable>