[media] v4l2-framework.txt: Update the locking documentation

This documents the new queue->lock and how to use it. It also removes the
documentation of v4l2_disable_ioctl_locking: this is only used in gspca and
will be removed once gspca has been converted to vb2.

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