> BTW, this parameter should be set during frame creation

I have put `gui_default_parameter` calls in the relevant *fns-files.
I ended up putting it in wherever the 'alpha' frame parameter is set.

> The resource and class names should then be documented under
> (emacs)Table of Resources in the user manual.

I've added it with a very brief description. I noticed that the alpha
frame parameter is not mentioned there, although it is also accessible
through an X resource. I took the liberty to add it there, also with a
very brief explanation. Do tell if it should be left for another commit.

I also notice that it would be very handy with some link to the lispref
describing the frame parameters from these sections, but I don't suppose
that's easily doable.