The documentation for AuthMechanism::MongoDbAws has a high-level description of the different methods of authenticating with AWS, but could use some more details about which variables need to be set for each method, the precedence order of the methods, etc. We should update the docs to include a section for each method we support. The obtaining credentials for AWS section of the spec can be used for inspiration: https://github.com/mongodb/specifications/blob/master/source/auth/auth.rst#id13