dillon1000/react

Fix ambiguity in doc comment for isValidElement (#12826)

`isValidElement(object)` checks if object is a ReactElement.

`@return {boolean} True if `object` is a valid component.` leading to confusion which was described in several blog posts:
- https://reactjs.org/blog/2015/12/18/react-components-elements-and-instances.html
- https://medium.com/@fay_jai/react-elements-vs-react-components-vs-component-backing-instances-14d42729f62