Document public API for DatabaseCleaner::Configuration

[etagwerker]

Hi there,

I think this can be a good issue for a first-time contributor. It would be great if we can increase documentation for the DatabaseCleaner::Configuration API: https://github.com/DatabaseCleaner/database_cleaner/blob/master/lib/database_cleaner/configuration.rb#L56-L72

Even though methods are pretty self-explanatory, I think it is a good idea to maintain a complete documentation for public methods. If you look at https://rubydoc.info/gems/database_cleaner/DatabaseCleaner/Configuration you will find some TODO comments but not much more than that.

In order to follow the standard defined by YARD, method documentation should follow this guide: https://rubydoc.info/gems/yard/file/docs/GettingStarted.md

If you think this is a good idea, we can create more issues for other classes that are poorly documented.

Best,
Ernesto