[media] videobuf2-memops.h: add to device-drivers DocBook

The comment metadata was wrong:

Warning(.//include/media/videobuf2-memops.h:25): cannot understand function prototype: 'struct vb2_vmarea_handler '
Warning(.//include/media/videobuf2-memops.h): no structured comments found

Fix and add to DocBook.

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