[kernel] fix comment in Log.register_category

src/kernel_services/plugin_entry_points/log.mli

@@ -240,8 +240,8 @@ module type Messages = sig
 
   val register_category: string -> category
   (** register a new debugging/verbose category.
-      Note: this should not be used directly by plug-in developers;
-      use instead [Plugin.S.Debug_category.add].
+      Note: categories must be added (e.g. via [add_debug_keys])
+      after registration.
       @since Fluorine-20130401
    *)