AEP-154: discuss what servers should do if it does not support preconditions.

[toumorokoshi]

#254 suggests that we should clarify that if a conditional header is not supported by the service, it should reject the request. This verbage is already present in AEP-154:

If any validator or conditional headers are supported for any operations in the
service, the use of unsupported conditional headers must result in an
error. (In other words, once a service gives the client reason to believe it
understands conditional headers, it must not ever ignore them.)

However, I'm not able to find this recommendation in RFC 9110, nor RFC 7232 which preceded it. I think it would be good to clarify the behavior, and perhaps upstream it to an RFC.