no-vary: URI/URL terminology

[reschke]

The introduction mixes "URI" and "URL"; that might be confusing to
readers. For URI, please cite RFC 3986. (maybe also explain why both
terms are needed)

"resource has multiple URLs" -> "when different URLs that only differ in
certain query components identify the same resource"

"some or all parts of the query" -> "...query component"

query parameter key needs a reference

"do not semantically affect the served resource" -> "do not affect what
resource is identified"

"raw output" -> "output" (maybe)

(see https://lists.w3.org/Archives/Public/ietf-http-wg/2025OctDec/0000.html)