Doc: Add Qt6 QDBus porting guide

Fixes: QTBUG-88027
Change-Id: Ic4fa2070aa7068a1d0ff02d43aa6a3d1d10db0d7
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

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

@@ -41,6 +41,26 @@
     In this topic we summarize those changes in Qt DBus, and provide
     guidance to handle them.
 
-    \section1 ADD STUFF HERE
+    \section1 The QtDBus module has undergone some basic consolidation and
+    cleanup. There should be minimal porting effort to be able to switch to Qt6.
 
+    \section2 QDBusMessage
+    The QDBusMessage::call() and QDBusMessage::asyncCall() methods with a fixed
+    size of arguments have been removed in Qt6.
+
+    \section2 QDBusArgument
+    The QDBusArgument::operator<<() specializations for QHash, QMap and QList
+    have been unified into common specializations for associative containers.
+
+    \section2 QDBusConnection
+    The QDBusConnection::sender() function has been removed in Qt6.
+
+    \section2 QDBusMessage
+    The QDBusMessage::createErrorReply() function now takes the QString \c name
+    argument by const reference instead of taking by value.
+
+    \section2 QDBusPendingReply
+    QDBusPendingReply has been changed to be a varadic template class in order
+    to support more than 8 template parameters now. The template parameters
+    are the types that will be used to extract the contents of the reply's data.
 */