serial: doc: .break_ctl() is called with port->mutex() held

Note that mutex_lock() should not be called with interrupts disabled.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/serial/driver

@@ -177,8 +177,7 @@ hardware.
 	should be terminated when another call is made with a zero
 	ctl.
 
-	Locking: none.
-	Interrupts: caller dependent.
+	Locking: caller holds port->mutex
 	This call must not sleep
 
   startup(port)