func_channel.c: Update MASTER_CHANNEL documentation

Be more explicit in what is meant by the master channel to eliminate
misunderstanding.

ASTERISK-23133

Change-Id: I453bcaf4b99404a5a3e345dbf093ac6c1afcfc72

funcs/func_channel.c

@@ -67,8 +67,11 @@
 			Gets or sets variables on the master channel
 		</synopsis>
 		<description>
-			<para>Allows access to the channel which created the current channel, if any.  If the channel is already
-			a master channel, then accesses local channel variables.</para>
+			<para>Allows access to the oldest channel associated with the current
+			channel if it still exists.  If the channel is the master channel or
+			the master channel no longer exists then access local channel variables
+			instead.  In other words, the master channel is the channel identified by
+			the channel's linkedid.</para>
 		</description>
 	</function>
 	<function name="CHANNEL" language="en_US">