QAbstractNativeEventFilter: document return value

(paragraph taken "as is" from the QObject::eventFilter documentation)

Change-Id: I1b7e92736103042d7105f1b26ba4784cbcc43dc2
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

src/corelib/kernel/qabstractnativeeventfilter.cpp

@@ -101,6 +101,10 @@ QAbstractNativeEventFilter::~QAbstractNativeEventFilter()
     On Blackberry (not plain QNX) \a eventType is set to "bps_event_t", and the \a message can be casted
     to a bps_event_t pointer.
 
+    In your reimplementation of this function, if you want to filter
+    the \a message out, i.e. stop it being handled further, return
+    true; otherwise return false.
+
     Example:
     \snippet code/src_corelib_kernel_qabstractnativeeventfilter.cpp 0
 */