gdkcursor: Add annotations for gdk_cursor_new_for_display()

It can fail on Wayland, and is also (transfer full).

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gdk/gdkcursor.c

@@ -226,7 +226,7 @@ gdk_cursor_get_cursor_type (GdkCursor *cursor)
  *
  * Creates a new cursor from the set of builtin cursors.
  *
- * Returns: a new #GdkCursor
+ * Returns: (nullable) (transfer full): a new #GdkCursor, or %NULL on failure
  *
  * Since: 2.2
  **/