Update the signal and slots docs

This lifts the documentation into a more recent context. One might
argue that Qt is a little bit older too.

Change-Id: I3e85f21bbad1179136629c2025e3499481fc8334
Task-number: QTBUG-16809
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

src/corelib/doc/src/objectmodel/signalsandslots.qdoc

@@ -49,17 +49,14 @@
     if a user clicks a \uicontrol{Close} button, we probably want the
     window's \l{QWidget::close()}{close()} function to be called.
 
-    Older toolkits achieve this kind of communication using
+    Other toolkits achieve this kind of communication using
     callbacks. A callback is a pointer to a function, so if you want
     a processing function to notify you about some event you pass a
     pointer to another function (the callback) to the processing
     function. The processing function then calls the callback when
-    appropriate. Callbacks have two fundamental flaws: Firstly, they
-    are not type-safe. We can never be certain that the processing
-    function will call the callback with the correct arguments.
-    Secondly, the callback is strongly coupled to the processing
-    function since the processing function must know which callback
-    to call.
+    appropriate. While successful frameworks using this method do exist,
+    callbacks can be unintuitive and may suffer from problems in ensuring
+    the type-correctness of callback arguments.
 
     \section1 Signals and Slots