ffplay.texi: document -codec

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffplay.texi

@@ -134,6 +134,8 @@ Exit when video is done playing.
 Exit if any key is pressed.
 @item -exitonmousedown
 Exit if any mouse button is pressed.
+@item -codec:@var{stream_type}
+Force a specific decoder implementation
 @end table
 
 @section While playing