[media] DocBook: media: update codec section, drop obsolete 'suspended' state

[media] DocBook: media: update codec section, drop obsolete 'suspended' state

The Codec section in the V4L2 specification was marked as 'suspended', even
though codec support has been around for quite some time. Update this
section, explaining a bit about memory-to-memory devices and pointing to
the MPEG controls section.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>