docs: copilot-instructions.md - Emphasize XML doc review

[VaclavElias]

PR Details

If we are going to use Copilot for reviews, it would be good if we could get also xml comments suggestions because these are vital for our game engine project.

This is the same PR as before (now closed), which was in the incorrect location in my local repo.

Related Issue

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.

docs: copilot-instructions.md - Emphasize XML doc review

• Instruct reviewers to always check for missing, incomplete, or incorrect XML documentation on public APIs.

PR Classification

Documentation update to improve code review guidelines.

PR Summary

This pull request updates copilot-instructions.md to strengthen requirements for XML documentation in code reviews. It introduces explicit instructions for reviewers to check and suggest XML comments on public APIs.

• copilot-instructions.md: Added detailed guidelines for reviewing, suggesting, and maintaining XML documentation comments on public types and members.