define decryption contract for multi-keyring

[lavaleri]

We want to be able to specify what security guarantees you can get from different keyrings (under certain conditions) as part of the Security Considerations section. Given the nature of multikeyrings, any guarantees will depend on security guarantees written for other keyrings.

Acceptance criteria:

• multikeyring security considerations includes security guarantees provided by multi-keyrings

[robin-aws]

Heavily dependent on #131