KeyIds definition is contradictory

[juneb]

The definition of key IDs in a KMS keyring is contraditory:
https://github.com/awslabs/aws-encryption-sdk-specification/blob/master/framework/kms-keyring.md#key-ids

• "Key IDs is a list of strings identifying KMS CMKs, in ARN format." This is true only for C.
• A CMK alias (e.g. "alias/MyCryptoKey") <<< not an ARN
• A key id, which is valid in some languages (not C), is missing.
• It does not use the correct KMS key ID format names (e.g. "alias name").
  https://docs.aws.amazon.com/kms/latest/developerguide/concepts.html#key-id
• It implies that only ARNs must be well-formed. Why would we ever accept anything that isn't well-formed?

The definition of Generator has a similar issue:

• The The string MUST be one of the following: list omits key ID, which is valid on encrypt in all languages except C.

[acioc]

PR merged. Tracking implementation in #129