Add missing documentation for QWaylandApplication::seat()

Documentation for the other functions refer to seat(), but that itself
isn't documented

Pick-to: 6.6
Change-Id: I9a628e87153b687b2fa444798de1af74e6251eee
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/platform/unix/qunixnativeinterface.cpp

@@ -271,6 +271,10 @@ QT_DEFINE_PRIVATE_NATIVE_INTERFACE(QEvdevKeyMapper);
     \fn wl_seat *QNativeInterface::QWaylandApplication::lastInputSeat() const
     \return the seat on which the last input event happened.
 */
+/*!
+    \fn wl_seat *QNativeInterface::QWaylandApplication::seat() const
+    \return the seat associated with the default input device.
+*/
 
 QT_DEFINE_NATIVE_INTERFACE(QWaylandApplication);