docs: clarify subscription completion result - #3171

Motivation and Context

Right now, the text calls the successful subscriptions/listen response "empty," but the result shown right below it includes resultType and the required subscription metadata. That could be interpreted as requiring a literal result: {} object, which would conflict with the schema and example.

This wording describes it as a completion result instead and clarifies that it contains no method-specific data beyond the standard result fields and subscription metadata.

How Has This Been Tested?

npm run prep

Breaking Changes

None.

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