doc: fix scrolling on iOS devices

Fixes an issue that prevented scrolling from going past large code
blocks on iOS devices. Also fixes a few minor styling issues that
came up in the discussion.

Fixes: https://github.com/nodejs/node/issues/5861
PR-URL: https://github.com/nodejs/node/pull/5878


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Lindstädt <robert.lindstaedt@gmail.com>