[DOC] Tap Dance - Refactor Tap Hold code sample

[iamdanielv]

Was getting confused between tap, hold, and held and how they were being used in code. I've renamed the variables and added comments to the sample to hopefully make it easier to understand how it works.

Description

I've refactored the sample for Tap Hold to use more descriptive variable names and added comments to hopefully make it easier to follow what is happening.

Types of Changes

• [ ] Core
• [ ] Bugfix
• [ ] New feature
• [ ] Enhancement/optimization
• [ ] Keyboard (addition or update)
• [ ] Keymap/layout/userspace (addition or update)
• [X] Documentation

Issues Fixed or Closed by This PR

N/A

Checklist

• [X] My code follows the code style of this project: C, Python
• [X] I have read the PR Checklist document and have made the appropriate changes.
• [X] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.
• [X] I have read the CONTRIBUTING document.
• [X] I have added tests to cover my changes.
• [X] I have tested the changes and verified that they work and don't break anything (as well as I can manage).