drm/writeback: wire drm_writeback.h to kernel-doc

drm_writeback.h included a lot of nice kernel-doc comments.
Wire it up so the header file is included in the kernel-doc
generated documentation.

Added a few simple comments to the two structs so they
get picked up by kernel-doc.

Signed-off-by: Sam Ravnborg <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Cc: Laurent Pinchart <[e-mail]>
Cc: Brian Starkey <[e-mail]>
Cc: Liviu Dudau <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Thomas Zimmermann <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/drm-kms.rst

···
 Writeback Connectors
 --------------------
 
+.. kernel-doc:: include/drm/drm_writeback.h
+  :internal:
+
 .. kernel-doc:: drivers/gpu/drm/drm_writeback.c
   :doc: overview
 

include/drm/drm_writeback.h

···
 #include <drm/drm_encoder.h>
 #include <linux/workqueue.h>
 
+/**
+ * struct drm_writeback_connector - DRM writeback connector
+ */
 struct drm_writeback_connector {
+	/**
+	 * @base: base drm_connector object
+	 */
 	struct drm_connector base;
 
 	/**
···
 	char timeline_name[32];
 };
 
+/**
+ * struct drm_writeback_job - DRM writeback job
+ */
 struct drm_writeback_job {
 	/**
 	 * @connector: