Document every function with an example

[baetheus]

1. All function or const arrow exports should have a short description and an example of use.
2. Don't bother turning const exports into function exports as we are going to tackle that in deno_doc over the summer
3. Document all algebraic structures with text meant to help the reader build an intuition for the structure.

Following are the files that need docs:

• [ ] Affect
• [ ] Array
• [ ] Const
• [ ] Datum
• [ ] Derivations
• [ ] Either
• [ ] Functions
• [ ] HKT
• [ ] Identity
• [ ] IO
• [ ] IOEither
• [ ] Map
• [ ] Monoid
• [ ] Nilable
• [ ] Option
• [ ] Ord
• [ ] Reader
• [ ] ReaderEither
• [ ] Record
• [ ] Semigroup
• [ ] Sequence
• [ ] Set
• [ ] Setoid
• [ ] State
• [ ] Task
• [ ] TaskEither
• [ ] These
• [ ] Tree
• [ ] Type Classes
• [ ] Types
• [ ] At
• [ ] Index
• [ ] Iso
• [ ] Lens
• [ ] Optional
• [ ] Prism
• [ ] Traversal
• [ ] Decoder
• [ ] Guard
• [ ] Schemable