Document the Client/ServerInterceptor requirements

[weissi]

Neither the doc comments ClientInterceptor docs nor the Interceptors Tutorial actually mention that you must forward the parts in exactly the same order as you received them which makes async interception hard.

See also #1618 / #1619 / #1181 and https://stackoverflow.com/questions/76534719/eventloopfuture-causing-invalid-state-unable-to-write-message/76535053#76535053 where this went wrong