Improve docstrings for Hooks

[neilramsay]

Hi there 👋,

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) documentation.

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

Cheers,
Neil