doc: update os.markdown

Some doc update based on improvement ideas I remember from when I used this module in node:

 - Mention windows returns 0 for `nice` values (which is obvious, but io makes no attempt to calculate something similar or return undefined, 0 is returned)
 - Mention platform and arch are aliases for `process` properties.
 - Document possible return values where appropriate, add examples in others.
 - Rename title in order to match other titles in the navigation.
 - Fix line that was over 80 characters long.

PR-URL: https://github.com/iojs/io.js/pull/976


Reviewed-By: Christian Tellnes <christian@tellnes.no>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>