Resolve issue #68

[juneb]

#68

Question: Should we define ARN as "Amazon Resource Name" or is that more confusing?

• Resolve contradiction: not always an ARN
• Remove well-formed. We would never accept anything that's not well-formed. This also eliminates the typo.
• Simplify the explanation of why we need key ARNs in the decryption keyring.

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[seebees]

This was resolved by #123