Add @todo to M999

Firmware/Marlin_main.cpp

@@ -8528,7 +8528,7 @@ Sigma_Exit:
 
     /*!
 	### M999 - Restart after being stopped <a href="https://reprap.org/wiki/G-code#M999:_Restart_after_being_stopped_by_error">M999: Restart after being stopped by error</a>
-    Usually doesn't work.
+    @todo Usually doesn't work. Should be fixed or removed. Most of the time, if `Stopped` it set, the print fails and is unrecoverable.
     */
     case 999:
       Stopped = false;