Add return value documentation and re-enable SA1615

[daviburg]

Problem

The .editorconfig suppresses SA1615 (element return value should be documented) and the .github/copilot-instructions.md says "Do NOT document return values". This convention is inherited from the internal BPM repo and is inappropriate for a public repository where XML docs surface in IntelliSense.

Proposed Changes

1. Update .github/copilot-instructions.md to remove the "Do NOT document return values" rule
2. Remove the dotnet_diagnostic.SA1615.severity = none suppression from .editorconfig
3. Add <returns> tags to all public methods that currently lack them
4. Review SA1623 (property summary wording) and SA1629 (doc ending with period) suppressions — re-enable if practical

Context

SA1615 was suppressed in the compliance PR (#13) because 15+ methods lacked return value docs. This issue tracks the proper fix: adding the docs rather than suppressing the rule.

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