doc: Add missing documentation for QXcbWindowFunctions::setWmWindowRole()

Change-Id: I5a457eb19fee011868cdf6d0534713c760fd57d2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: David Faure <david.faure@kdab.com>

src/platformheaders/xcbfunctions/qxcbwindowfunctions.qdoc

@@ -85,6 +85,35 @@
     relayed to the function retrieved by QGuiApplication.
 */
 
+/*!
+    \typedef QXcbWindowFunctions::SetWmWindowRole
+    \since 5.6.2
+
+    This is the typedef for the function returned by
+    QGuiApplication::platformFunction when passed the
+    value returned by setWmWindowRoleIdentifier().
+*/
+
+/*!
+    \fn QByteArray QXcbWindowFunctions::setWmWindowRoleIdentifier()
+    \since 5.6.2
+
+    This function returns the byte array that can be used to query
+    QGuiApplication::platformFunction to retrieve the SetWmWindowRole function.
+*/
+
+/*!
+    \fn void QXcbWindowFunctions::setWmWindowRole(QWindow *window, const QByteArray &role)
+    \since 5.6.2
+
+    Sets the WM_WINDOW_ROLE property from \role on the corresponding
+    X11 window.
+
+    This is a convenience function that can be used directly instead
+    of resolving the function pointer. \a window and \a role will be
+    relayed to the function retrieved by QGuiApplication.
+*/
+
 /*!
     \typedef QXcbWindowFunctions::SetWmWindowIconText