[media] DocBook: mention that EINVAL can be returned for invalid menu indices

The documentation suggested that if the control value is wrong only
ERANGE can be returned. But in some cases (an invalid menu index)
EINVAL can also be returned. Clarify this.

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