Tidy up NKRO_ENABLE rules

[fauxpark]

Description

Removing outdated, copypasted comments linking to TMK documentation; rules.mk should avoid links that may die in the future (ie. Docsify/Gitbook).

I also considered modifying the comment to mention the NK_TOGG keycode, as enabling the feature does not immediately make QMK send reports over the NKRO interface... however, again, I decided this is really a job for the docs to explain better.

Types of Changes

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

Issues Fixed or Closed by This PR

Checklist

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

[tzarc]

Well this was a fun review.