Clarified how to show feature values in waterfall plots

[martinrohbeck]

Motivation and Context

This is a small edit, but it helps users understand how to display a feature’s value in a waterfall plot without having to dig through the whole plotting code.

Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

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How Has This Been Tested?

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Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• An entry has been added to CHANGELOG.md (if relevant for users).
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

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