QtIFW

Doc: fixed links to QMessageBox documentation

In Qt 5, you have to specify the index files for
the Qt 5 modules that you want to link to for QDoc
to find them. If you add the brackets in the end of
the method name, QDoc creates the links automatically
and you do not need the \l command.

Change-Id: Ia3dd78bb17e9c176747d406216930295c9fa02a8
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>