clarify revocation support for short-lived certificates

[bdaehlie]

We should make sure that our text regarding revocation accurately reflects the ways in which revocation support for short-lived certs will differ from what we do for 90-day certs.

[aarongable]

For example, if we do not support revoking short-lived certificates at all, then

When key compromise is demonstrated, ISRG blocks the key from use in future issuance and revokes all unexpired certificates that used that key.

needs to be updated to specify "...and revokes all unexpired long-lived certificates...".