docs(hooks.md): clarify useDispatch, see #1468

[papb]

This PR clarifies the memoization of useDispatch and how the generated dispatch function can be safely used as a dependency of an useCallback hook.

I wrote this thinking of what I would have liked to read earlier today. This little clarification would have saved some time.

Thanks for considering!

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This would resolve #1468 (in my opinion) if it wasn't already closed.

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[timdorr]

Thanks!

[markerikson]

Unfortunately, I think this might have the same problem we've been running into with the versioning: it only updates the file under /docs, and not in the versioned folders, so it'll only show up under the "Next" version instead of the main page.

[papb]

@markerikson Hmm, should I create mirror PRs targeting other branches? Which ones?

[markerikson]

It's not "other branches" - it's the files under https://github.com/reduxjs/react-redux/tree/master/website/versioned_docs .