[media] DocBook: Remove a warning at videobuf2-v4l2.h

There's no need to document to_foo() macros, and the macro is
badly documented anyway. That removes this warning:
	include/media/videobuf2-v4l2.h:43: warning: No description found for parameter 'vb'

While here, remove the parenthesis for container_of(). The
countainer_of() already have parenthesis inside it.

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