doc: use blockquotes for Stability: markers

Use blockquotes instead of code blocks for stability markers in
the docs. Doing that:

- Makes the makers appear correctly when viewed e.g. on github.
- Allows remark-lint rules like `no-undefined-references` to work
  properly (https://github.com/nodejs/node/pull/7729).

PR-URL: https://github.com/nodejs/node/pull/7757


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>