[media] buffer.rst: clarify how V4L2_CTRL_FLAG_MODIFY_LAYOUT/GRABBER are used

Explain when the V4L2_CTRL_FLAG_MODIFY_LAYOUT and
V4L2_CTRL_FLAG_MODIFY_GRABBER flags should be used.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>