[media] DocBook media: clarify BGR666

The documentation is not clear whether this is a three or four byte
format. Clarify this.

Also move the BGR666 format to the other 32 bit formats.

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