media: extended-controls.rst: add note to the MPEG2 state controls

Add a note mentioning that these two controls are not part of the
public API while they still stabilizing.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>