bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Eli Zaretskii]

> Cc: 37774@debbugs.gnu.org, juri@linkov.net
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 9 Dec 2019 17:24:19 +0200
> 
> >    M-x load-file RET foo.elc RET
> >    M-x ert-run-tests-interactively RET RET
> 
> Ok, thanks. It'll be better to use some safer faces, though. Or even 
> define those inside the test file.

If we have a special test theme, we could use any face name we want.

> > We could say something like "'region' and some others".  Not saying
> > that some have this by default would be a mistake, IMO.
> 
> The original wording implies an exhaustive list. How would you like it 
> to be phrased now?

How about the below?

  "By default, only a small number of faces, notably, @code{region},
  have this attribute set."