drm/shmem-helper: Correct doc-comment of drm_gem_shmem_get_sg_table()

drm_gem_shmem_get_sg_table() never returns NULL on error, but a ERR_PTR.
Correct the doc comment which says that it returns NULL on error.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -654,7 +654,8 @@ EXPORT_SYMBOL(drm_gem_shmem_print_info);
  * drm_gem_shmem_get_pages_sgt() instead.
  *
  * Returns:
- * A pointer to the scatter/gather table of pinned pages or NULL on failure.
+ * A pointer to the scatter/gather table of pinned pages or an ERR_PTR()-encoded
+ * error code on failure.
  */
 struct sg_table *drm_gem_shmem_get_sg_table(struct drm_gem_shmem_object *shmem)
 {
@@ -680,7 +681,8 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_get_sg_table);
  * drm_gem_shmem_get_sg_table() should not be directly called by drivers.
  *
  * Returns:
- * A pointer to the scatter/gather table of pinned pages or errno on failure.
+ * A pointer to the scatter/gather table of pinned pages or an ERR_PTR()-encoded
+ * error code on failure.
  */
 struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_shmem_object *shmem)
 {