Add warning message to ExperimentalMcpApi annotation and change level (#398)

- add the warning message to `ExperimentalMcpApi` annotation
- change level from error to warning

## Types of changes
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [x] I have added or updated documentation as needed

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/ExperimentalMcpApi.kt

@@ -35,7 +35,10 @@ import kotlin.annotation.AnnotationTarget.VALUE_PARAMETER
  * - Property setters
  * - Type aliases
  */
-@RequiresOptIn
+@RequiresOptIn(
+    message = "This API is experimental. It may be changed in the future without notice.",
+    level = RequiresOptIn.Level.WARNING,
+)
 @MustBeDocumented
 @Target(
     CLASS,