Documentation update for ABC base classes

[pipermerriam]

What is wrong?

Previous to https://github.com/ethereum/py-evm/pull/1805 we had classes like eth.vm.BaseVM that served as the reference. Now, we have interfaces defined in eth.abc.

There are now two primary problems.

• Many of our docstrings might still refer to the wrong class. Most class references in docstrings should be referencing the abc base classes now.
• Most of the methods and properties on our abc base classes don't have docstrings.

How can it be fixed

Need to update existing docstrings to point to the right classes and add docstrings to all of the methods in the eth.abc directory.

Bonus points for documenting the class properties too.

[pipermerriam]

I labeled this as Good First Issue though it may require some domain knowledge to properly write many of the docstrings.