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 <jani.nikula@intel.com>
Acked-by: Maíra Canal <mcanal@igalia.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113101100.1373856-1-mripard@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>

Author: mripard

Documentation/gpu/drm-internals.rst

@@ -208,6 +208,13 @@ follows:
 	``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
 ===================