More docs for prelude libraries

WIP

Tried to add code examples for each function. They might not have the best style, or you may know  easier to read ones, suggestions are welcome.

Also: Is it legal to lift descriptions from Haskell docs?
Yes, but "overloaded" is ambiguous because it can mean either a type class method or a straight-up ambiguous name. I'd say "a user-supplied equality predicate instead of the Eq instance"