On this page:
https://secure.phabricator.com/book/phabricator/article/arcanist_lint_unit/
There is a link to "Adding New Classes" that currently points here:
https://secure.phabricator.com/diviner/find/?book=contributor&name=Adding_New_Classes&type=article&jump=1
This returns a 404.
Instead, it should be pointing here:
https://secure.phabricator.com/book/phabcontrib/article/adding_new_classes/