Update the QSignalBlocker docs to show actual behavior

QSignalBlocker resets to the previous state, which might have already
been a blocked signal state.

Task-number: QTBUG-39352
Change-Id: I918cc6ea346755b940e027cee5d6704824fbba32
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/kernel/qobject.cpp

@@ -513,7 +513,7 @@ void QMetaCallEvent::placeMetaCall(QObject *object)
     \code
     const bool wasBlocked = someQObject->blockSignals(true);
     // no signals here
-    someQObject->blockSignals(false);
+    someQObject->blockSignals(wasBlocked);
     \endcode
 
     except the code using QSignalBlocker is safe in the face of