Improve warnings about refs on functional components during runtime and in docs

[pke]

I had trouble to get ref working on one of my composite components.

This inline ref function

<MyComp ref={element => this.listElement = element}/>

will always be called with element===null on mount and unmount the like. Thats because MyComp is a stateless/functional component.

react should print a warning if one tries to use ref with stateless components.

[stephenkingsley]

I think stateless component just a tool, so in my options, this is unnecessary.

[Maximilianos]

This issue can be closed because I believe it has been resolved by: #8635

[aweary]

@Maximilianos thanks!