Provide additional information about verbosity for Buck commands
Provide additional information about verbosity for Buck commands

Summary:
- Verbosity levels go from `1` to `8`, but Buck currently doesn't distinguish between all the adjacent levels.
- Higher levels include all output associated with lower levels.
- Output is affected by factors other than just the verbosity level.
- Provide a table of the levels, the corresponding boolean functions in the source, and a brief description for each level.
- Provide guidance about how to search the source for a more precise understanding of the behavior of a specific level.

Reviewed By: sbalabanov

fbshipit-source-id: 04aea3804c

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