Add doc explaining the gotcha when connecting a signal to qApp->exit

Change-Id: I981e4bfdf679bf755665748e9d3b389a94561e55
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/corelib/doc/snippets/code/src_corelib_kernel_qcoreapplication.cpp

@@ -56,7 +56,7 @@ QApplication::sendEvent(mainWindow, &event);
 
 //! [1]
 QPushButton *quitButton = new QPushButton("Quit");
-connect(quitButton, SIGNAL(clicked()), &app, SLOT(quit()));
+connect(quitButton, SIGNAL(clicked()), &app, SLOT(quit()), Qt::QueuedConnection);
 //! [1]
 
 

src/corelib/kernel/qcoreapplication.cpp

@@ -1370,6 +1370,13 @@ void QCoreApplicationPrivate::execCleanup()
   By convention, a \a returnCode of 0 means success, and any non-zero
   value indicates an error.
 
+  It's good practice to always connect signals to this slot using a
+  \l{Qt::}{QueuedConnection}. If a signal connected (non-queued) to this slot
+  is emitted before control enters the main event loop (such as before
+  "int main" calls \l{QCoreApplication::}{exec()}), the slot has no effect
+  and the application never exits. Using a queued connection ensures that the
+  slot will not be invoked until after control enters the main event loop.
+
   Note that unlike the C library function of the same name, this
   function \e does return to the caller -- it is event processing that
   stops.
@@ -1898,6 +1905,13 @@ void QCoreApplicationPrivate::maybeQuit()
     to quit(), and you also often connect e.g. QAbstractButton::clicked() or
     signals in QAction, QMenu, or QMenuBar to it.
 
+    It's good practice to always connect signals to this slot using a
+    \l{Qt::}{QueuedConnection}. If a signal connected (non-queued) to this slot
+    is emitted before control enters the main event loop (such as before
+    "int main" calls \l{QCoreApplication::}{exec()}), the slot has no effect
+    and the application never exits. Using a queued connection ensures that the
+    slot will not be invoked until after control enters the main event loop.
+
     Example:
 
     \snippet code/src_corelib_kernel_qcoreapplication.cpp 1