bug#55623: 29.0.50; Mention that (face-foreground 'default) can return "unspecified-fg"

[Visuwesh <visuweshm@gmail.com>]

[புதன் மே 25, 2022] Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" wrote:

> Visuwesh <visuweshm@gmail.com> writes:
>
>> In a tty frame and when using a theme that does not explicitly set the
>> default face's :foreground/:background [1], (face-attribute 'default :foreground)
>> returns "unspecified-fg".  This value is surprising when the docstring
>> of `face-attribute' says,
>>
>>     To ensure that the return value is always specified and absolute, use a
>>     value of ‘default’ for INHERIT; this will resolve any unspecified or
>>     relative values by merging with the ‘default’ face (which is always
>>     completely specified).              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>     ^^^^^^^^^^^^^^^^^^^^^
>>
>> I opened the Elisp manual and tried to isearch for "unspecified-fg", but
>> it got me no matches.  It would be nice if this return value was
>> documented somewhere.
>
> Isn't that a special color which means to use the terminal's default
> foreground (and/or background, in the case of unspecified-bg) colors?
>
> If so, it should be documented as that instead.

Indeed, "unspecified-fg/bg" means to use the terminal's default fg/bg.
But AFAICT, it is not specified in the manual anywhere.