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>