bug#54156: [External] : bug#54156: 28.0.91; set-face-attribute with a nil FRAME doesn't change the default

[Drew Adams]

> The default attributes for faces that are used when creating new
> frames are stored in face--new-frame-defaults.  When a frame is
> created, those attributes are merged with what the face's spec (from
> defface) says.  Thus, having 'unspecified' in face--new-frame-defaults
> for an attribute has no effect: the definition of the attribute in
> defface will override it.
> 
> So we need a special trick to override defface with 'unspecified',
> and that trick is this call:
>   (set-face-attribute 'region t :background 'unspecified)
> This is handled specially in internal-set-lisp-face-attribute to
> do what Damien wants.

This is likely all documented.  However, it's a
bit tricky to absorb, and perhaps to explain.

Consider taking another look at the doc for this,
and see if you think you could improve it to make
this a bit clearer.

(Just a suggestion.  Pretty much any text can be
improved, and rereading is the way to begin.)