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

[Eli Zaretskii <eliz@gnu.org>]

> From: Visuwesh <visuweshm@gmail.com>
> Cc: luangruo@yahoo.com,  adam@alphapapa.net,  55623@debbugs.gnu.org
> Date: Wed, 25 May 2022 22:52:00 +0530
> 
> How about the following instead then?
> 
>     The 'default' face is always fully specified except in special cases
>     of TTY frames where :foreground and :background attributes may be
>     the strings "unspecified-bg" and "unspecified-bg" respectively to
>     mean to use the TTY's color for the foreground and background.

This is inaccurate and thus misleading.  These special color names are
just like any other color names, they are "special" only when Emacs
needs to actually use them on the screen.  For any other purposes,
they are just color names.  Thus, the default face is "fully
specified" even when these colors are used.  Also, these colors can be
used by other faces, not just by 'default'.

Technically, these colors just tell Emacs not to emit a color-changing
command when it writes text to the screen, or emit a command that
tells the terminal driver "reset to your default color".  But this is
an implementation detail, and we cannot talk about it in the manual
without explaining a lot of details about the inner workings of color
support on TTY frames.