Clarify `host_gui->request_show()` when the plugin's gui isn't created yet

[abique]

I think the host could try to create the gui then show it.
TODO..

[baconpaul]

I think that's reasonable; the only thing that comes to mind is : is there a way for the host to report that it can't actually do that? Like request_show has a synchronous return value etc....

so thinking about what happens if you do a request show and a create fails would be the thing I'd like to document.

"request-show and i didn't because the user had never opened the ui before" does seem like odd behavior I agree.