Consider documenting that `rvk` denotes expiry for Coze keys. about coze

[zamicol]

Revision:

Revoke

A Coze key may be revoked by signing a coze containing the field rvk with an
integer value greater than 0. For example, the integer value 1 is suitable
to denote revocation. The current Unix timestamp is the suggested value for
rvk.

Example Self Revoke

{
 "pay": {
  "alg": "ES256",
  "iat": 1623132000,
  "msg": "Posted my private key online",
  "rvk": 1623132000,
  "tmb": "cLj8vsYtMBwYkzoFVZHBZo6SNL8wSdCIjCKAwXNuhOk",
  "typ": "cyphr.me/key/revoke"
 },
 "sig": "KVjPjMVHoL828WyAH5biqIOt-IOaQ5EBtN_7eQifP2w3agUHu6KfqO40_oqQ5GE_BShgXvhbK0O6Z2h5YPNAcw"
}

• rvk - Unix timestamp of key expiry.

Coze explicitly defines a self-revoke method so that third parties may revoke
leaked keys. Systems storing Coze keys should provide an interface permitting a
given Coze key to be marked as expired by receiving a self-revoke message.
Self-revokes with future times must immediately be considered as expired.

rvk and iat must be a positive integer less than 2^53 – 1
(9,007,199,254,740,991), which is the integer precision limit specified by
IEEE754 minus one. Revoke checks must error if rvk is not an integer or larger
than 2^53 - 1.

Key expiration policies, key rotation, backdating, and alternative revocation
methods are outside the scope of Coze.

[zamicol]

Fixed by 5af99fd