drm/docs: Explicitly document default CRTC background behavior

Add a paragraph explaining that the default behavior for areas which
are not covered by planes or where planes are blending with the CRTC
background, is black.

This is alluded to in the "pixel blend mode" property docs, but not
called out explicitly.
Reviewed-by: NSimon Ser <contact@emersion.fr>
Signed-off-by: NSean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230103152738.1213785-1-sean@poorly.run

drivers/gpu/drm/drm_plane.c

@@ -46,6 +46,11 @@
  * properties that specify how the pixels are positioned and blended, like
  * rotation or Z-position. All these properties are stored in &drm_plane_state.
  *
+ * Unless explicitly specified (via CRTC property or otherwise), the active area
+ * of a CRTC will be black by default. This means portions of the active area
+ * which are not covered by a plane will be black, and alpha blending of any
+ * planes with the CRTC background will blend with black at the lowest zpos.
+ *
  * To create a plane, a KMS drivers allocates and zeroes an instances of
  * &struct drm_plane (possibly as part of a larger structure) and registers it
  * with a call to drm_universal_plane_init().