docs: Improve the documentation around useFetcher to explain that fetcher.submit is stable across rerenders

[julienw]

This was implemented originally in https://github.com/remix-run/react-router/pull/10336.

It wasn't clear to me from that PR if fetcher.load itself is stable too, but maybe it is?

Happy to make any needed changes!

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[julienw]

I'm awaiting the permission to sign the CLA.