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

[Mattias Engdegård]

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.

Proposed patch attached.

0001-Add-font-lock-doc-markup-face.patch
Description: Binary data