Porting Guide: Mention changed behavior of QCApplication::quit()

Mention change of behavior introduced by 89f7a2759c6b in the porting
documentation.

Change-Id: I3c282362f5852cc7768e6655fc7b3901d68f2b10
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
(cherry picked from commit 3cbe11f93475a845e4e7849e1315570ea782e595)

src/corelib/doc/src/qt6-changes.qdoc

@@ -847,4 +847,16 @@
     Note that if your MyEvent class allocates memory (e.g. through a
     pointer-to-implementation pattern), then you will have to implement
     custom copy semantics.
+
+    \section1 Other classes
+
+    In Qt 5, QCoreApplication::quit() was equivalent to calling
+    \l{QCoreApplication::exit()}. This just exited the main event loop.
+
+    In Qt 6, the method will instead try to close all top-level windows by posting
+    a close event. The windows are free to cancel the shutdown process by
+    ignoring the event.
+
+    Call QCoreApplication::exit() to keep the non-conditional behavior.
+
 */