doc/ffplay, ffplay: add information regarding volume control

ffplay now supports dynamic volume control. This documents the supported
behavior.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

doc/ffplay.texi

@@ -197,6 +197,15 @@ Toggle full screen.
 @item p, SPC
 Pause.
 
+@item m
+Toggle mute.
+
+@item 9, 0
+Decrease and increase volume respectively.
+
+@item /, *
+Decrease and increase volume respectively.
+
 @item a
 Cycle audio channel in the current program.
 

ffplay.c

@@ -3681,6 +3681,9 @@ void show_help_default(const char *opt, const char *arg)
            "q, ESC              quit\n"
            "f                   toggle full screen\n"
            "p, SPC              pause\n"
+           "m                   toggle mute\n"
+           "9, 0                decrease and increase volume respectively\n"
+           "/, *                decrease and increase volume respectively\n"
            "a                   cycle audio channel in the current program\n"
            "v                   cycle video channel\n"
            "t                   cycle subtitle channel in the current program\n"