BKE_lib_remap: add comment note about some remapping options.

Fact that those options are only used in a specific case, and that the
same behavior is ensured in a different part of the code in other cases,
is fairly confusing and unfortunate... At least document it.

source/blender/blenkernel/BKE_lib_remap.h

@@ -182,11 +182,18 @@ typedef enum IDRemapperApplyOptions {
    *
    * For remapping the old ID users will be decremented and the new ID users will be
    * incremented. When un-assigning the old ID users will be decremented.
+   *
+   * NOTE: Currently unused by main remapping code, since usercount is handled by
+   * `foreach_libblock_remap_callback_apply` there, depending on whether the remapped pointer does
+   * use it or not. Need for rare cases in UI handling though (see e.g. `image_id_remap` in
+   * `space_image.c`).
    */
   ID_REMAP_APPLY_UPDATE_REFCOUNT = (1 << 0),
 
   /**
    * Make sure that the new ID datablock will have a 'real' user.
+   *
+   * NOTE: See Note for #ID_REMAP_APPLY_UPDATE_REFCOUNT above.
    */
   ID_REMAP_APPLY_ENSURE_REAL = (1 << 1),