Doc: add clarification about signals not being buffered when blocked

Task-number: QTBUG-15238
Change-Id: Id762007415245f104ffe5cdfd100889f9a73ae95
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

src/corelib/kernel/qobject.cpp

@@ -1385,6 +1385,8 @@ bool QObject::eventFilter(QObject * /* watched */, QEvent * /* event */)
     Note that the destroyed() signal will be emitted even if the signals
     for this object have been blocked.
 
+    Signals emitted while being blocked are not buffered.
+
     \sa signalsBlocked()
 */