drm: Document requirements for driver-specific KMS props in new drivers

When extending support for a driver-specific KMS property to additional
drivers, we should apply all the requirements for new properties and
make sure the semantics are the same and documented.

v2: devs of the driver which introduced property shall help and ack

Signed-off-by: Sebastian Wick <sebastian.wick@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240410122008.38207-1-sebastian.wick@redhat.com