doc: fix, modernize examples in docs

* Use single quotes consistently
* Modernize examples to use template strings and arrow funcs
* Fix a few typos
* Example edits for consistency

PR-URL: https://github.com/nodejs/node/pull/4282


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>