dillon1000/react

Docs: Clarification of setState() behavior (#9329)

* Clarification of setState() behavior

`setState()` is a frequent source of confusion for people new to React, and I believe part of that is due to minimization of the impact of the asynchronous behavior of `setState()` in the documentation. This revision is an attempt to clarify that behavior. For motivation and justification, see [setState Gate](https://medium.com/javascript-scene/setstate-gate-abc10a9b2d82).

* Update reference-react-component.md

* Signature fix

* Update to address @acdlite concerns

* Add more details