Create custom instructions for GH copilot to review ref docs (Azure#28858)

Author: isra-fel

.github/instructions/reference-doc.instructions.md

@@ -0,0 +1,9 @@
+---
+applyTo: "**/help/*.md"
+---
+
+## Reference help documentation requirements
+
+When writing reference documents for PowerShell cmdlets, please follow these guidelines to ensure readability and helpfulness:
+
+1. **Avoid over-simplified description** - For parameters and cmdlets, avoid over-simplified description such as "Some help", or simply repeating the parameter name.