Provide documentation for data types

[ochrons]

Many existing Cats data types are not mentioned at all on the http://non.github.io/cats//datatypes.html page. Of those that are mentioned, only FreeMonads and State have reasonable documentation.

[adelbertc]

Leaving a checklist here that can be edited as people leave comments on which ones they're taking up to avoid duplicate work:

• Const - https://github.com/non/cats/pull/445
• OneAnd - https://github.com/non/cats/pull/706
• Xor - https://github.com/non/cats/pull/451
• EitherT - Add EitherT docs #1854
• Ior - Add docs for Ior #1822
• Validated - https://github.com/non/cats/pull/444
• Kleisli: https://github.com/non/cats/pull/440

[ceedubs]

All of the data types in #430 (comment) now have docs. I'm going to close this out and we can open new tickets for anything that we find is missing.