The mutation classes are a vital component to Django Evolution, and are

some of the oldest parts of code. Unfortunately, they completely lacked

documentation, aside from a comment here or there. To help keep the code

manageable and understandable to those who haven't touched it before,

this change adds documentation to all the classes and functions (except

those that will be removed/modified in an upcoming change), detailing

the purpose, arguments, return types, and exceptions raised.

No code changes, but unit tests pass anyway.