docs: clarify JSX equality

[vib3-ch3ck]

#7751

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[rickhanlonii]

This is not correct. JSX is never "equal" because it returns new objects every time. The purity aspect mentioned in the docs talk about it returning the "same JSX" which is more about when the component runs, the same thing is returned every time, so it doesn't depend on something outside that could change it.

It's not really about JSX so much as the same output, and that doesn't mean you should "construct" and equal output (in that case you would need to know how to construct it to make it equal), it should execute the same code paths and return the same output each time.