docs: Add explicit MUST NOT for notification response

[chr-hertel]

Rephrasing the sentence about Notification response. According to JSON RPC spec the server MUST NOT send a response.

Motivation and Context

I'm currently working on an SDK and was a bit confused about the wording here, but the original JSON RPC spec helped: https://www.jsonrpc.org/specification#notification

How Has This Been Tested?

not applicable?

Breaking Changes

This is only about being more explicit in the docs - I don't think this is a breaking change.

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

Additional context