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>

doc/api_assets/style.css

@@ -339,6 +339,8 @@ p code,
 li code {
   font-size: 0.9em;
   color: #040404;
+  background-color: #f2f5f0;
+  padding: 0.2em 0.4em;
 }
 
 span.type {