Update feature_macros.md

[Buttenup]

Added a link to the usage of SAFE_RANGE as this wasn't clear to me when I visited this page in isolation.

Description

I came here recently to create some simple macros on my new board, the usage of SAFE_RANGE wasn't clear to me and omitting it leads to errors that seem unrelated when you don't know what it's for. I added a note that it should be used and a link to the page that explains it more clearly.

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

Checklist

• 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.
• My change requires a change to the documentation.
• 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).