drm/doc: Document KUnit expectations

The DRM and KMS frameworks and helpers gain more and more kunit
coverage, so let's document what our expectations are.

Suggested-by: Jani Nikula <[e-mail]>
Acked-by: Maíra Canal <[e-mail]>
Acked-by: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Acked-by: Jani Nikula <[e-mail]>
Acked-by: Simona Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Maxime Ripard <[e-mail]>

Documentation/gpu/drm-internals.rst

···
 	``CONFIG_VIRTIO_UML`` and ``CONFIG_UML_PCI_OVER_VIRTIO`` are not
 	included in it because they are only required for User Mode Linux.
 
+KUnit Coverage Rules
+~~~~~~~~~~~~~~~~~~~~
+
+KUnit support is gradually added to the DRM framework and helpers. There's no
+general requirement for the framework and helpers to have KUnit tests at the
+moment. However, patches that are affecting a function or helper already
+covered by KUnit tests must provide tests if the change calls for one.
 
 Legacy Support Code
 ===================