docs: improve redis commands documentation

[akrantz01]

Required for all PRs:

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format (e.g. feat: or fix:)

Updates the documentation for redis to clearly specify the allowed result types for commands and explain the format of the command array. Also errors when an unknown result type is specified rather than defaulting to a string.

[telegraf-tiger]

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