Add more information about the value field under Input in the API doc (modelcontextprotocol#164)

<!-- Provide a brief summary of your changes -->
Add more information about the value field under Input in the API doc.

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
the descrioption for
[value](https://github.com/modelcontextprotocol/registry/blob/81e5174a593a7488715b0bf0ae77a4dc466b4d35/docs/openapi.yaml#L219)
is not clearly.

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
No need.

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
No

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] 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)
- [ x ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [ x ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ x ] My code follows the repository's style guidelines
- [ x ] New and existing tests pass locally
- [ x ] I have added appropriate error handling
- [ x ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

Author: luoxiner

docs/openapi.yaml

@@ -219,7 +219,7 @@ components:
         value:
           type: string
           description: |
-            The default value for the input. If this is not set, the user may be prompted to provide a value.
+            The default value for the input. If this is not set, the user may be prompted to provide a value. If a value is set, it should not be configurable by end users.
 
             Identifiers wrapped in `{curly_braces}` will be replaced with the corresponding properties from the input `variables` map. If an identifier in braces is not found in `variables`, or if `variables` is not provided, the `{curly_braces}` substring should remain unchanged.
         is_secret: