https://github.com/mongodb/bson-ruby/blob/master/README.md:

The [API Documentation](https://api.mongodb.com/bson-ruby/current/) is located at api.mongodb.com.

The api.mongodb.com subdomain is no longer valid

Current documentation can be found [here](http://docs.mongodb.org/ecosystem/tutorial/ruby-bson-tutorial/#ruby-bson-tutorial).

This will automatically redirect to https://www.mongodb.com/docs/ruby-driver/current/tutorials/bson-v4/#ruby-bson-tutorial, however should be updated to reflect the final url.