v3.2: Consider moving guidance on supplying a retrieval URL (and other parts of the spec that justify various features) to the Learn site

[handrews]

From @notEthan in PR #4389:

I think this goes somewhat into the weeds of circumstances when an implementation might need to be told the retrieval URI. But it seems to me that if there is an intended retrieval URI, well, the implementation always needs to be told that. If the implementation is itself retrieving the document from that URI, it must be given that URI for actual retrieval. If the implementation isn't doing the retrieval or is retrieving from a different URI, whatever the reason, it must be given the nominal retrieval URI to use for resolution. Network configuration, test environment, etc are relevant considerations for retrieval, but regardless how those are set up, the base URI has to be given.

Where "this" means the list of use cases for accepting a user-supplied base URI.

The reason that list is present is that, over the past ten years of dealing with this issue for both JSON Schema and OAS, there is tremendous pushback and lack of understanding if the requirement is given without justification.

We could move all such justification to the Learn site, but for now it is in the spec so that implementors understand why they are being asked to support this.