confbridge: Clarify mute sound documentation.

The mute/unmute sounds are only played when the
action is initiated using the DTMF menu.

ASTERISK-24756

Change-Id: I55b3dd5bc166096bf5e2f547ddd0ce355f36e3dc

configs/samples/confbridge.conf.sample

@@ -243,8 +243,8 @@ type=bridge
                 ; left the conference. This is used for user intros.
                 ; Example "_____ has left the conference"
 ;sound_kicked ; The sound played to a user who has been kicked from the conference.
-;sound_muted  ; The sound played when the mute option it toggled on.
-;sound_unmuted  ; The sound played when the mute option it toggled off.
+;sound_muted  ; The sound played when the mute option is toggled on using DTMF menu.
+;sound_unmuted  ; The sound played when the mute option is toggled off using DTMF menu.
 ;sound_only_person ; The sound played when the user is the only person in the conference.
 ;sound_only_one ; The sound played to a user when there is only one other
                 ; person is in the conference.