Slight clarification of LED/RGB Matrix custom effect docs

[fauxpark]

Description

This part of the docs is a little confusing as to what the led/rgb_matrix_mode() line is for, and is kind of in the middle of the section regarding the .inc file.

The LED Matrix docs, at least this area, seems to be not quite in syn with the RGB Matrix docs so I also updated it to match.

Types of Changes

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

Issues Fixed or Closed by This PR

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.
• [ ] 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).