Raw AES Keyring doc security considerations

Related to #187

- IV exhaustion/how many keys can we wrap this way?
- wrapping key use; both how many times it is used to encrypt data keys and where/whether/how it is used elsewhere.
- Does not write any information about what wrapping algorithm suite was used to wrap the data key.
- Puts data that doesn't belong in the provider info, into the provider info

Start by reading related issue #187 and reviewing the four listed concerns about IV exhaustion, wrapping-key reuse, missing algorithm-suite information, and provider info. Done means documenting the security considerations and clarifying what data belongs in provider info.

Written by the indexing model from the issue text.