docs: Document nullable MCP Tool output schemas

[sweat123]

What is the purpose of the change

Fixes #1140.

Update the next-version documentation for nullable fields in MCP Tool
outputSchema.

This documents the functionality introduced for
alibaba/nacos#15675 and
implemented by
alibaba/nacos#15698.

Brief changelog

• Document the Nullable option for MCP Tool output fields.

• Explain that a nullable string field is saved as:

  {
    "type": ["string", "null"]
  }
  

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