Documents CHANNEL(musicclass) taking priority over m([x]) in waitExten

If waitExten specifies a music class to use with its music on hold option, it will use
CHANNEL(musicclass) instead if that channel variable has been set on the initiating
channel.  This documents that behavior in the waitExten app so that this can be known
without checking the documentation of the code in function local_ast_moh_start.

(closes issue ASTERISK-18804)
........

Merged revisions 347239 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@347240 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/pbx.c

@@ -671,7 +671,8 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 					<option name="m">
 						<para>Provide music on hold to the caller while waiting for an extension.</para>
 						<argument name="x">
-							<para>Specify the class for music on hold.</para>
+							<para>Specify the class for music on hold. <emphasis>CHANNEL(musicclass) will
+							be used instead if set</emphasis></para>
 						</argument>
 					</option>
 				</optionlist>