Document that React.render() may return null if component doesn't have an instance/state

[jimfb]

Assuming we keep this behavior, it's worth noting in the documentation that React.render() may return null in cases where the component doesn't have any instance state.

[zpao]

(removing "good first bug" since this would be best done by somebody with more knowledge about React)

[blainekasten]

Looks like this can be closed since #4964 was merged and is marked as fixing this issue. cc @zpao

[jimfb]

Correct, thanks!