drm/doc: rename FB_DAMAGE_CLIPS section

Make it more human-readable.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201216202222.48146-2-contact@emersion.fr