Review `nonNull`, add documentation

[sjshuck]

Pun intended. 🙂

I appreciate this warning about filtered. (I guess the reason the type is written as Optic' instead of Prism' is to discourage people from reviewing it?)

I'm sure there's a reason nonNull is an improper Prism' instead of an improper Traversal', I'm just not sure what the reason is. Maybe the fix is to document wherever optics laws are violated in the library, as lens tries to, not only here but

>>> let s = "-0" in over _Number id s == s
False

etc.