bus: add comment to explain assert

src/libsystemd/sd-bus/sd-bus.c

@@ -2897,7 +2897,7 @@ static int bus_process_internal(sd_bus *bus, bool hint_priority, int64_t priorit
 
         /* We don't allow recursively invoking sd_bus_process(). */
         assert_return(!bus->current_message, -EBUSY);
-        assert(!bus->current_slot);
+        assert(!bus->current_slot); /* This should be NULL whenever bus->current_message is */
 
         BUS_DONT_DESTROY(bus);