All of Mongoid's configuration options are thoroughly documented in comments, in Mongoid::Config. We're duplicating those documents in the template file for the mongoid.yml, so that users can see what options are available to adjust there.

We ought to have a single source of truth for documentation like this. By scraping the options and the comments from the mongoid/config.rb file, we can dynamically build the available options and their documentation in the mongoid.yml template.