[media] videobuf2-v4l2: document two helper functions

Document vb2_ops_wait_prepare() and vb2_ops_wait_finish(),
in order to fix those two warnings:
	Documentation/media/kapi/v4l2-dev.rst:166: WARNING: c:func reference target not found: vb2_ops_wait_prepare
	Documentation/media/kapi/v4l2-dev.rst:166: WARNING: c:func reference target not found: vb2_ops_wait_finish

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>