Add doctests (excellent first-time issue)

Doctests are a great way to provide compilable and testable example code for scaladoc. We should cover as many methods with doctests as possible. Here is an example of PR adding doctests. Note that to test the doctests run sbt coreJVM/test This issue encompasses the effort of adding more doctest coverage to our type classes and data types. It would be a great first-time contribution.

If you are interested, please comment here on what type classes or data types you'd like to work on.