bug#57499: Documentation bug in the docstring of set-face-attribute?

[Gregory Heytings]

> It isn't deprecated, it's invalid.  There's no need to have anything 
> like this in NEWS, because all our documentation says the color 
> attributes can have only string values (in addition to 'unspecified').

Okay (I thought you wanted to retain the "undocumented but not really 
invalid" use of nil).

> Warning: nil value for `%s' is invalid, use 'unspecified instead.

I added the name of the face in the message, which is I think useful to 
locate the cause of the warning.

Should this go to master or to emacs-28?

Display-a-warning-for-some-uses-of-nil-in-face-attri.patch
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