clarify naming of AWS KMS keyring

[mattsb42-aws]

We currently just call the AWS KMS keyring the "KMS keyring". AWS KMS is not the only thing called "KMS". We should be explicit in our naming.

[juneb]

Please add a 'doc impact' label to this issue.

[mattsb42-aws]

Updating here: we agreed that we want to do this prior to releasing keyrings in Java and Python.