doc: fenced all code blocks, typo fixes

This changes the code blocks from 4-space indentation to ``` fences for
better syntax highlighting and future linting support. Minor On-the-fly
changes for typos and highlight breaking markdown have been made.

JSON-Style objects have been changed so their closing bracket is
on the same line as the opening one.

Known issues:
* Not every JSON / object notation has been improved. Should
  make another run for this.
* Some example functions break hightlighting due to various
  combinations of brackets. However changing them means leaving
  the code style.

Fixes: https://github.com/nodejs/node/issues/4726
PR-URL: https://github.com/nodejs/node/pull/4733


Reviewed-By: Roman Reiss <me@silverwind.io>