drm: clarify and linkify DRM_CLIENT_CAP_WRITEBACK_CONNECTORS docs

Make it clear that the client is responsible for enabling ATOMIC
prior to enabling WRITEBACK_CONNECTORS. Linkify the reference to
ATOMIC.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/434200/