Improve zap grpc logger documentation on verbosity levels

[chrisdoherty4]

The documentation around verbosity levels for gRPC logging is unclear - this change aims to rectify that.

The original issue (#221) contains some conflating information regarding how to use the verbosity levels that people keep landing on.

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[chrisdoherty4]

@googlebot I signed it!

[yashrsharma44]

Hi @chrisdoherty4 , is it ready for review?

[chrisdoherty4]

@yashrsharma44 yes. Its minor, I know. I just got frustrated having to dig into code when this simple comment would've answered my question.

[yashrsharma44]

Thanks for the changes!