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

Note that mutex_lock() should not be called with interrupts disabled.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/serial/driver b/Documentation/serial/driver
index 3b08df5..09e73e0 100644
--- a/Documentation/serial/driver
+++ b/Documentation/serial/driver
@@ -177,8 +177,7 @@
 	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)