bug#50041: Add font-lock-doc-markup-face

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Fri, 13 Aug 2021 12:53:04 +0200
> 
> Add a standard face for mark-up syntax and keywords inside code documentation (doc comments/strings) whose body text is typically set in font-lock-doc-face. This could include various tags, brackets, quotes, keywords or other syntactically special characters.
> 
> Justification:
> 
> - A standard face means that users and theme authors can set it once for all modes needing it.
> - Having a face that harmonises with font-lock-doc-face reduces the risk of bad ergonomics and aesthetics.
> - Setting doc mark-up in a distinct face makes in-source documentation more readable.
> - It also aids writing documentation by providing feed-back to the programmer in the same way as fontifying regular language constructs.
> 
> The CC modes have the foresight of already using `font-lock-doc-markup-face` if defined, anticipating its eventual addition, defaulting to `font-lock-constant-face`. I therefore propose that the new face will inherit from `font-lock-constant-face` by default.

Which one of these justifies having these faces in font-lock.el, as
opposed in markdown mode itself, or in rst.el?  It is unusual to have
mode-specific faces in a general-purpose Lisp file, let alone a
preloaded one.