Isomorphism lenses should be better explained

[jackfirth]

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Docs should mention what an isomorphism actually is, and how isomorphism lenses are different from normal lenses in that they don't "zoom in" to view a small piece, they view the whole object in a different form.

[AlexKnauth]

This seems like a cop-out, and it is, but I didn't try to put a proper explanation of isomorphisms in there because I had no idea what an isomorphism was until you told me. So I thought you should be the one to explain them in the docs.

[jackfirth]

That's quite alright, this is definitely something I plan to do myself. When I do though I'd like you to review it thoroughly to let me know if the explanation is grok-able.