Documentation update for ABC base classes

[simon-saliba]

What was wrong?

FIX: https://github.com/ethereum/py-evm/issues/1806

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 was it fixed?

Added docstrings for non-documented abstract methods in abc.py. Did not find any docstring that refer to the old classes. This was probably fixed in other PRs since this issue dates from a long time ago.

Todo:

• [x] Clean up commit history

• [ ] Add entry to the release notes

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