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

Documentation/gpu/drm-kms.rst

@@ -496,6 +496,13 @@ addition to the one mentioned above:
 
 * An IGT test must be submitted where reasonable.
 
+For historical reasons, non-standard, driver-specific properties exist. If a KMS
+driver wants to add support for one of those properties, the requirements for
+new properties apply where possible. Additionally, the documented behavior must
+match the de facto semantics of the existing property to ensure compatibility.
+Developers of the driver that first added the property should help with those
+tasks and must ACK the documented behavior if possible.
+
 Property Types and Blob Property Support
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