Fix awkward paragraph in 8.2.4.5 "Loading a Reference Schema"

[handrews]

In section 8.2.4.5, the 2nd paragraph reads:

When schemas are downloaded, for example by a generic user-agent that doesn't know until runtime which schemas to download, see Usage for Hypermedia (Section 11).

which I had trouble parsing just now while referring someone to that section. I think what's going on here is that:

• We want to acknowledge that while implementations are not required to automatically fetch referenced schemas, sometimes that will be a thing that happens.
• In particular, acknowledging systems where the exact schemas needed are only known at runtime.
• Section 11 is specifically about ways to fetch schemas that allow meta-data that can work with RFC 8288 links and/or media types and their parameters. This is relevant, but not the only option- loading from files is also common, for example.

We should figure out what we're trying to say here and say it more clearly, without assuming HTTP or similar protocols.

[Relequestual]

@handrews Had a go at fixing this.
To be honest, I didn't find it confusing at first read anyway, but hey ho.

[Relequestual]

Closed in favour of #849