Add more detailed API information to API section in README, provide mock keyring and backends implementation

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Currently, I am writing tests which need a mock keyring. Looking at this project, the README seems to be the primary or only documentation for using this module. I see a few areas which would improve my experience in using this module. These are expanding the API documentation to include type annotations and more detailed explanations and constraints for parameters, a reference to or provided mock for keyring using monkey patching, and a reference to or provided mock for a backend generator able to provide multiple mock backends.

This is with acknowledgement that [pytest-keyring](https://pypi.org/project/pytest-keyring/) appears insufficient for my needs.

Start with the README, which is identified as the primary documentation for this module, and review the tests that need a mock keyring. Compare the requested API detail with pytest-keyring, then define what documentation, a monkey-patchable mock keyring, and a mock backend generator supporting multiple backends would need to provide.

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