[Docs]: Docs for shouldRevalidate make it sound like it is only for opting out

[steinybot]

Describe what's incorrect/missing in the documentation

The docs for shouldRevalidate only mention the case for opting out of data revalidation. From my investigation and testing it also supports the opposite case of triggering revalidation when the default would have been false. It should say that you can also opt-in when the default is false.

This is relevant to react-router-relay where we need to force revalidation when the relay environment changes and the queries have been disposed of. I had a question from a maintainer of react-router-relay asking whether this was a suitable usage of shouldRevalidate. I'm pretty sure that it is but it would be much more reassuring if the docs mention it and we can rely on it staying that way.