docs: add additionalProperties to input schema specification (#1630)

The `additionalProperties` is completely missing in the input schema
specification.
![Screenshot 2025-06-11 at 14 28
46](https://github.com/user-attachments/assets/e2ef9a46-d621-4690-8114-6d563987d6c2)

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Co-authored-by: Michał Olender <[email protected]>

Author: mfori

sources/platform/actors/development/actor_definition/input_schema/specification.md

@@ -106,6 +106,7 @@ If you switch the input to the **JSON** display using the toggle, then you will
 | `schemaVersion` | Integer | Yes | The version of the input schema <br/>specification against which <br/>your schema is written. <br/>Currently, only version `1` is out. |
 | `properties` | Object | Yes | This is an object mapping each field key <br/>to its specification. |
 | `required` | String | No | An array of field keys that are required. |
+| `additionalProperties` | Boolean | No | Controls if properties not listed in `properties` are allowed. Defaults to `true`. <br/>Set to `false` to make requests with extra properties fail. |
 
 :::note Input schema differences