drm: Improve dumb callback docs

Noticed while reviewing a patch from Eric. Also add a todo for the
dumb_map_offset callbacks (it should be simple to do, but piles of
work). Plus fix up vbox, because vbox.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Fabio Rafael da Rosa <fdr@pid42.net>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181127091921.8325-1-daniel.vetter@ffwll.ch

include/drm/drm_drv.h

@@ -527,8 +527,10 @@ struct drm_driver {
 	 * @dumb_map_offset:
 	 *
 	 * Allocate an offset in the drm device node's address space to be able to
-	 * memory map a dumb buffer. GEM-based drivers must use
-	 * drm_gem_create_mmap_offset() to implement this.
+	 * memory map a dumb buffer.
+	 *
+	 * The default implementation is drm_gem_create_mmap_offset(). GEM based
+	 * drivers must not overwrite this.
 	 *
 	 * Called by the user via ioctl.
 	 *
@@ -548,6 +550,9 @@ struct drm_driver {
 	 *
 	 * Called by the user via ioctl.
 	 *
+	 * The default implementation is drm_gem_dumb_destroy(). GEM based drivers
+	 * must not overwrite this.
+	 *
 	 * Returns:
 	 *
 	 * Zero on success, negative errno on failure.