Improve confusing "Relative References in URIs" / "Relative References in URLs" section titles and wording

[handrews]

These sections are quite hard to parse. The URIs section applies to all forms of $ref and operationRef. The URLs section applies to URLs in the Server (and by extension, Paths) Objects. Some other places are unclear (see #1701, for example).

[handrews]

PR merged to 3.1.1. We'll sync up 3.2.0 after 3.1.1 and 3.0.4 are more-or-less done.