Fix docs on ReactElement

[jddxf]

We should test $$typeof field against REACT_ELEMENT_TYPE to check if something is a React Element.

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[gaearon]

Thanks for the PR but I believe the original is clearer. REACT_ELEMENT_TYPE is Symbol.for('react.element'). Cheers!

[jddxf]

REACT_ELEMENT_TYPE is a plain number if there is no native Symbol nor polyfill.

[gaearon]

Right, but this is a fallback case and doesn't seem as important to document here to me.
Maybe could add "or a magic number as a fallback".

[jddxf]

Got it. Then I'll just ignore it. Thanks.