Update copilot-instructions: allow return value documentation

## Problem

The `.github/copilot-instructions.md` currently says "Do NOT document return values" (no `` tag). This convention is inherited from the internal BPM repo and is inappropriate for a public samples repository where XML docs help developers understand the code.

## Proposed Changes

1. Update `.github/copilot-instructions.md` to remove the "Do NOT document return values" rule

## Context

The Samples repo has minimal public API surface, so the code change scope is small — primarily the copilot-instructions update.

See also: Azure/Connectors-NET-SDK#41

Open .github/copilot-instructions.md and locate the rule that prohibits documenting return values. Remove that rule while preserving the surrounding instructions; done means the file no longer forbids return-value documentation. No test or run command is mentioned in the issue.

索引モデルが issue の本文から書いたものです。