drm: re-add deleted doc for drm_gem_open_ioctl

Add back the removed documentation for drm_gem_open_ioctl.
This patch is submitted in response to [1].

[1] https://lore.kernel.org/linux-arm-msm/20200728085244.GY6419@phenom.ffwll.local/Signed-off-by: NSteve Cohen <cohens@codeaurora.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1596000952-27621-1-git-send-email-cohens@codeaurora.org

drivers/gpu/drm/drm_gem.c

@@ -871,6 +871,9 @@ drm_gem_flink_ioctl(struct drm_device *dev, void *data,
  * @file_priv: drm file-private structure
  *
  * Open an object using the global name, returning a handle and the size.
+ *
+ * This handle (of course) holds a reference to the object, so the object
+ * will not go away until the handle is deleted.
  */
 int
 drm_gem_open_ioctl(struct drm_device *dev, void *data,