docs: root state type with TypeScript and configureStore

[firstred]

• Closes Describe inferring RootState type from store.getState #324

When passing the root reducers directly to configureStore(), it is not possible to get the root state type directly from the reducers. For the type, you will then have to access store.getState() instead.

So, instead of type RootState = ReturnType<typeof rootReducer> you will have to use type RootState = ReturnType<typeof store.getState>. This PR adds this little piece of information.

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[markerikson]

The info is just a bit off. The problem comes when you don't define the root reducer explicitly, and instead pass the slice reducers to configureStore(), like:

const store = configureStore({
    reducer: {
        first: firstReducer,
        second: secondReducer,
    }

});

There's no rootReducer to reference, so we have to refer to store.getState instead.

Can you tweak the wording and example to match that?

[firstred]

I am going to familiarize myself a bit more with the material before I continue with this PR. I thought I knew a lot about the Redux Starter Kit / Toolkit already, but today showed that it still had a few surprises for me.
Thank you for being so patient and helpful. It's much appreciated!!

[markerikson]

Sure, and thanks for putting the PR together!

[firstred]

I just updated the PR, taking this feedback into account:

• No rootReducer to refer: Describe inferring RootState type from store.getState #324 (comment)
• Possible TS circular reference when adding middleware: docs: root state type with TypeScript and configureStore #326 (comment)

[markerikson]

This PR needs to be updated to take into account the changes from #549 and #559 .