Document that values are immutable but not persistent

[mfikes]

CLJS Bean values are immutable. This lets identical? work on them along with other guarantees you gain via immutability. (Reinforce that passed JavaScript maps and arrays should be effectively immutable to ensure this.)

But the object are not persistent. (Operations like assoc involve CoW.)

It is worth documenting (perhaps in the Object Extraction section?) some of these aspects.