The link placed on "API documentation is also available" in the README on github master points to https://api.mongodb.com/mongoid/current/

This however returns an Access Denied error with the following XML:

<Error>
<Code>AccessDenied</Code>
<Message>Access Denied</Message>
<RequestId>A0HY41CRVG9NXPDJ</RequestId>
<HostId>
5EXT/uGn51HrQyQMI3UAPc0Re81zR/QTDMxMTIFTHBOCpPwtdBLIVZBaF6JGhq9rDbsdYy4SJOw=
</HostId>
</Error>

Looks like either the link needs to be updated or the server configuration fixed.