bug#47012: xref copies keymap properties to minibuffer

[Eli Zaretskii]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Wed, 31 Mar 2021 18:47:30 +0300
> Cc: 47012@debbugs.gnu.org
> 
> (defface xref-file-header '((t :bold t :inherit compilation-info))
>    "Face used to highlight file header in the xref buffer."
>    :version "27.1")

As we all know, faces can be customized.  If some attributes are
"magic", in that the display will be hard to read if those attributes'
values are changed, then I think we should document that in the
respective doc strings.