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.