[media] DocBook: fix most of warnings at videobuf2-core.h

[media] DocBook: fix most of warnings at videobuf2-core.h

	include/media/videobuf2-core.h:112: warning: No description found for parameter 'get_dmabuf'
	include/media/videobuf2-core.h:146: warning: No description found for parameter 'm'
	include/media/videobuf2-core.h:146: warning: Excess struct/union/enum/typedef member 'mem_offset' description in 'vb2_plane'

There are still several warnings, but those are hard to fix,
as they're actually a bug at DocBook. Those should be fixed on
separate patches.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>