<!-- [rfced] Section 15.3.7. In the following sentence, "Date",
"ETag", "Content-Location", and "Vary" have links in the HTML and
PDF outputs. Would you like to add cite tags for those header
fields not defined in this document (Cache-Control and Expires)?
Current:
A server that generates a 206 response MUST generate the following
header fields, in addition to those required in the subsections
below, if the field would have been sent in a 200 (OK) response to
the same request: Date, Cache-Control, ETag, Expires,
Content-Location, and Vary.
-->
I don't think xml2rfc allows us the same kind of linking to externals; so I'd prefer to leave things as they are (otherwise this gets too distracting).