Improve doc clarity around TypeScript and `createAsyncThunk` usage

[EvHaus]

I had a difficult time figuring out how to get proper TypeScript responses for dispatched thunk actions. I read the docs several times, and even though the answer was there (use type narrowing via match), I missed it each time.

I think I skipped it because the relevant information was nested under the "Manually defining thunkAPI types" category, which I wasn't doing in my case.

Since this type narrowing is very useful and relevant even when you're not manually typing thunkAPI, I'm making a proposal in this PR to move this information slightly higher in the doc and give it a dedicated section.

For more context, see my question on Discord.

I hope you'll consider making this small change. Thank you!