This PR adds a small note to clarify that tools passed to the sampling request don't need to be registered.

Motivation and Context

We talked a bit in discord and, at least to me, it wasn't clear if the specification required the tool specified in the createMessage request to also be registered on the server.

How Has This Been Tested?

Not tested as is just a doc changes.

Breaking Changes

Nope just a docs 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