Improve docstrings for Hooks

Hi there :wave:,

It would be nice to have Python docstrings included on the public interfaces when developing hooks.
Would pull requests for docstrings be welcome?

For example, after `cfn init` for hooks, we have a `../handlers.py` that looks similar to this:

In Visual Studio Code, when I hover over `cloudformation_cli_python_lib` specific classes/properties I don't get any information for what they are, or what to expect.

I couldn't find out what all the `request.hookContext` properties mean, and what `dict` keys are available on `request.hookContext.targetModel` from the [Implement Hook handlers (Python)](https://docs.aws.amazon.com/cloudformation-cli/latest/hooks-userguide/hooks-model.html#model-hook-project-code-handler-python) documentation.

In the case of `HookContext` some docstrings like this would be welcome:

Cheers,
Neil