Update documentation for policy and relay testing guides

- Added a closing section to the POLICY_USAGE_GUIDE.md to emphasize the benefits of the policy system for implementing complex relay behavior.
- Included a closing section in the RELAY_TESTING_GUIDE.md to summarize the guide's purpose and its support for testing complex Nostr protocol features.

docs/POLICY_USAGE_GUIDE.md

@@ -689,3 +689,4 @@ Migrate custom validation logic to policy scripts:
 ```
 
 The policy system provides a flexible, maintainable way to implement complex relay behavior while maintaining performance and security.
+

docs/RELAY_TESTING_GUIDE.md

@@ -615,3 +615,4 @@ When adding new features that require multi-relay testing:
 - [cmd/relay-tester/README.md](../../cmd/relay-tester/README.md) - Protocol testing
 
 This guide provides the foundation for testing complex Nostr protocol features that require multiple relay coordination. The testing infrastructure is designed to be extensible and support various testing scenarios while maintaining reliability and performance.
+