Improve Virtual Keyboard documentation

The documentation being referred to is:
https://doc.qt.io/qt-6/qtvirtualkeyboard-deployment-guide.html#integration-method
particularly about the desktop integration.

I think that is not true. If the existing application is not configured to use virtual keyboard, it has to be recompiled to enable it. As far as I see from my test on desktop Windows, linking to virtual keyboard and setting environment variable are necessary (that is what I mean by "configured to use virtual keyboard" previously). It is not like that virtual keyboard just pops up magically for any existing application.

The only exception here is probably the virtual keyboard implemented on server side in a Wayland session. But still, I think it is better to clarify it.