Description of Ed25519-SHA256-RFC5705 does not match C tor.

Found by @dgoulet.

In our spec, we say that the TLS key exporter should be called with its context field set to the initiator's KP_relayid_ed. In reality, the C Tor implementation uses CID, which is SHA256(KP_relayid_rsa) for initiator.

Since it doesn't really matter what the context is here, we don't need to create a new authentication format... but we do need to document the one we have.