Added command-line option explanations and usage examples for CLI scripts

[michizhou]

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Fixes issue #763:

Description of the changes being introduced by the pull request:
Updated the docstrings of the CLI script files repo.py and client.py with in-depth explanations of command-line options for the sake of completeness and in order to improve the user experience. Also included a few more concrete usage examples for command-line options in client.py.

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[awwad]

Thanks for the help! LGTM.

I'll leave 763 open, since repo.py still lacks docstrings for some of its functions.