Document the fact that we switched to using boto3 to fetch credentials, which includes getting credentials in a different search order and in more locations than was previously used.  Point to Credentials documentation and describe how to avoid using the }default credentials if desired:

You can set AWS_PROFILE="" in your shell or add os.environ["AWS_PROFILE"] = "" to your script. Alternatively, you can create an AWS profile specifically for your Atlas credentials and set AWS_PROFILE to that profile name. We'll update the docs accordingly.

These changes should be in the PyMongo Auth AWS readme, the PyMongo 4.3 changelog, and in the authentication example in the PyMongo docs.