[media] videobuf2-core: Add it to device-drivers DocBook

Most of the stuff at videobuf2-core are ok for adding it to
DocBook.

Two notes here:
1) As videobuf2-core will be soon be changed, better to
   not spend too much efforts right now, as things will
   change soon;

2) struct vb2_queue has a number of private elements that are
   documented. As Kernel nano documentation format handles
   "private:" arguments, we need to put them on a separate
   comment block or to remove. Keeping the comments is
   obviously better ;)

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>