bug#13503: 24.3.50; option `scroll-bar-adjust-thumb-portion'

[Drew Adams]

> > Where is it?  What is it?
> >  
> > It is not autoloaded.  It is called out in NEWS.  Grepping 
> > Lisp sources shows that it is mentioned only in cus-start.el.
> > Where is the doc?
> 
> Some variables and functions are defined in C, so it's not enough to
> grep Lisp files.  This particular variable is defined in xterm.c.

Yes, I assumed it was in C code (which I do not have).

> Since it's only available on X (based on features that need X), you
> don't see its doc string on Windows.

Windows users should either not have the variable at all or they should see its
doc, which should let them know that it is not usable on Windows.  It is wrong
for them to have the variable and not be able to see its doc.

And NEWS should call out the fact that it is only for Emacs that uses X Window.
We should not be announcing new options that (some, in this case Windows) users
cannot find anything out about.

We currently say nothing at all about this option in NEWS, except that it is
new.  In that case, (all) users must be able to get info about it from Emacs in
some other way, e.g., C-h v or a manual.