v3.2: Consider additional guidance around `$self` usage

[handrews]

From @karenetheridge in PR #4389

Perhaps we can simplify this by simply strongly recommending that $self is an absolute URI (i.e. fully resolved; resolving it against another base URI is a no-op), and point to the RFC to explain why.

"this" refers to the guidance on how to resolve relative $self values, which walks through the RFC3986§5.1.1-4 options to explain the process, since older OAS versions just mentioned the retrieval URI (§5.1.3) and not the steps in §5.1.2 or §5.1.4.

I would love to just say "resolve per RFC3986" but in my experience that is not sufficient for many people to implement it properlly. However, I would be happy to kick that content over to the learn site if there is consensus to do so, perhaps along with the further $self recommendations that @karenetheridge is suggesting.

I do think that anytihng we can do to streamline the actual specification in favor of moving explanations to the Learn site would be a good thing. I have tended to put more in the spec because historically our spec explains a lot, but I'd support moving in the other direction.

[handrews]

We did end up streamlining the guidance substantially when PR #4389 was rewritten as PR #4556, which was then merged.