[Docs]: improve usage of second arg of `useSelector` (equality function)

The documentation talks about using shallowEqual as second argument for comparison (or lodash), although the documentation doesn’t explain what are the arguments of this function and how a user could write their own custom equality function.

Is there a way to improve that part of the documentation?

[markerikson]

@dwaltrip : tbh it’s a trivial enough function that there’s never been a specific need to document it 😃 (Plus in this case you don’t normally call it directly, you just import it and pass it as the second arg to useSelector.)

But yeah, happy to have a PR!

[ematipico]

I understand that shallowEqual per se accepts two properties, but when used with useSelector, what’s inside left and right?

By doing some logging, I can see left and right are the portion of the state selected by the selector.