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/

include/uapi/drm/drm.h

@@ -812,8 +812,8 @@ struct drm_get_cap {
  * DRM_CLIENT_CAP_WRITEBACK_CONNECTORS
  *
  * If set to 1, the DRM core will expose special connectors to be used for
- * writing back to memory the scene setup in the commit. Depends on client
- * also supporting DRM_CLIENT_CAP_ATOMIC
+ * writing back to memory the scene setup in the commit. The client must enable
+ * &DRM_CLIENT_CAP_ATOMIC first.
  */
 #define DRM_CLIENT_CAP_WRITEBACK_CONNECTORS	5