clarify rules on modifying auth-info for intermediaries

[reschke]

Raised by Amos in https://lists.w3.org/Archives/Public/ietf-http-wg/2015JanMar/0452.html:

Section 3 paragraph 3 says "Intermediaries are not allowed to modify the field value in any way."

RFC 7235 uses wording in the form: "A proxy forwarding ... MUST NOT modify ..."

I believe the Authentication-Info should share both normative MUST NOT,
and term "proxy" instead of intermediary. Since there are legitimate
cases where gateways and/or other intermediaries may need to change it
per the relevant auth scheme.