chathistory: Specify that REDACT in chathistory does not depend on event-playback

[progval]

As pointed out in #537

[jwheare]

The list of "eithers" is a bit awkward with repeated bits. What about this instead?

When a message is redacted, [`chathistory`][] responses SHOULD either:

* Exclude it entirely
* After the redacted message, include a `REDACT` message in the response that doesn't count towards message limits. The redacted message's content and/or tags MAY be replaced with a placeholder.

The `draft/event-playback` capability is not required in order to include `REDACT` messages in `chathistory` responses.

[progval]

Done, with a grammar fix