ffplay: update and extend documentation for channel and stream switching

Signed-off-by: Marton Balint <cus@passwd.hu>

doc/ffplay.texi

@@ -174,13 +174,16 @@ Toggle full screen.
 Pause.
 
 @item a
-Cycle audio channel.
+Cycle audio channel in the curret program.
 
 @item v
 Cycle video channel.
 
 @item t
-Cycle subtitle channel.
+Cycle subtitle channel in the current program.
+
+@item c
+Cycle program.
 
 @item w
 Show audio waves.

ffplay.c

@@ -3500,9 +3500,9 @@ void show_help_default(const char *opt, const char *arg)
            "q, ESC              quit\n"
            "f                   toggle full screen\n"
            "p, SPC              pause\n"
-           "a                   cycle audio channel\n"
+           "a                   cycle audio channel in the current program\n"
            "v                   cycle video channel\n"
-           "t                   cycle subtitle channel\n"
+           "t                   cycle subtitle channel in the current program\n"
            "c                   cycle program\n"
            "w                   show audio waves\n"
            "s                   activate frame-step mode\n"