Clarify default value in ElicitResult documentation

[timheuer]

Updated XML documentation to clarify default value for action.

Motivation and Context

In the IntelliSense and other documentation it clarifies what any default value would be if not set.

Breaking Changes

No, documentation only

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)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[mikekistler]

Looks good! 👍

Thanks for improving our docs!