Improve documentation

[milesfrain]

This library would greatly benefit from a guide that describes how to use unfoldr. The current explanation is pretty terse.

I was initially led here from Data.Map.Internal.toUnfoldable, which was pretty confusing. It also took me a while to realize that the argument to go is a "todo list" of subtrees to process.

Some resources that I found helpful to get more comfortable with unfoldr are:

• https://hackage.haskell.org/package/base-4.12.0.0/docs/Data-List.html#g:9
• https://jordanmartinez.github.io/purescript-jordans-reference-site/content/21-Hello-World/04-Collections-and-Loops/src/21-Unfoldable.html from @JordanMartinez

I'd like to incorporate some of that content (and perhaps some additional examples and explanation) into the official library docs.