[media] V4L2 Spec: improve the G/S_INPUT/OUTPUT documentation

As Rupert pointed out, the phrase "It is good practice" should be replaced
with "You must".

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Rupert Eibauer <Rupert.Eibauer@ces.ch>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>