doc: improve addons.markdown copy

General improvements to the documentation in addons.markdown.

PR-URL: #4320
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

Author: jasnell