Guidance for using RawInfo in protocol schemas

[LPardue]

The main schema defines RawInfo and our QUIC and HTTP/3 schemas use it. However, while implementing the update to latest drafts and migrating code, I wasn't always sure how to deal with things like STREAM frame RawInfo::length vs. RawInfo::payload_length. If things are ambiguous, we risk some interoperability issues. I think it would help to be quite explicit on how to populate the raw fields of protocol elements such as frames.