drm: writeback: document callbacks

Document the callbacks:
    drm_connector_helper_funcs.prepare_writeback_job
    drm_connector_helper_funcs.cleanup_writeback_job

The documentation was pulled from the changelong introducing the
callbacks, originally written by Laurent.

Adding the missing documentation fixes the following warnings:
drm_modeset_helper_vtables.h:1052: warning: Function parameter or member 'prepare_writeback_job' not described in 'drm_connector_helper_funcs'
drm_modeset_helper_vtables.h:1052: warning: Function parameter or member 'cleanup_writeback_job' not described in 'drm_connector_helper_funcs'

v2:
  - Fix formatting (Daniel)
  - Drop changelog text and add reference (Daniel)
  - Improve grammar. and use "operation" (Laurent)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406194746.26433-3-sam@ravnborg.org

include/drm/drm_modeset_helper_vtables.h

@@ -1075,8 +1075,35 @@ struct drm_connector_helper_funcs {
 	void (*atomic_commit)(struct drm_connector *connector,
 			      struct drm_connector_state *state);
 
+	/**
+	 * @prepare_writeback_job:
+	 *
+	 * As writeback jobs contain a framebuffer, drivers may need to
+	 * prepare and clean them up the same way they can prepare and
+	 * clean up framebuffers for planes. This optional connector operation
+	 * is used to support the preparation of writeback jobs. The job
+	 * prepare operation is called from drm_atomic_helper_prepare_planes()
+	 * for struct &drm_writeback_connector connectors only.
+	 *
+	 * This operation is optional.
+	 *
+	 * This callback is used by the atomic modeset helpers.
+	 */
 	int (*prepare_writeback_job)(struct drm_writeback_connector *connector,
 				     struct drm_writeback_job *job);
+	/**
+	 * @cleanup_writeback_job:
+	 *
+	 * This optional connector operation is used to support the
+	 * cleanup of writeback jobs. The job cleanup operation is called
+	 * from the existing drm_writeback_cleanup_job() function, invoked
+	 * both when destroying the job as part of an aborted commit, or when
+	 * the job completes.
+	 *
+	 * This operation is optional.
+	 *
+	 * This callback is used by the atomic modeset helpers.
+	 */
 	void (*cleanup_writeback_job)(struct drm_writeback_connector *connector,
 				      struct drm_writeback_job *job);
 };