doc: copyedit setTimeout() documentation

Copyedit the documentation for setTimeout() and enforce wrapping at 80
characters in the markdown file for nearby text.

PR-URL: https://github.com/nodejs/node/pull/4434


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: Stephan Belanger <admin@stephenbelanger.com>