doc: made code spans more visible in the API docs

This makes the code spans in the API docs more visible and
therefore readable by adding some background color.

PR-URL: https://github.com/nodejs/node/pull/3573


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>