doc: improvements to debugger.markdown copy

General improvements to debugger.markdown

PR-URL: https://github.com/nodejs/node/pull/4436


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>