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