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: Simon Ser <contact@emersion.fr>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230103152738.1213785-1-sean@poorly.run