feat: add multi parameters

docs/variable.rst

@@ -123,6 +123,12 @@ Parameters
      - The value of the variable is a list.
 
        **Default value**: `false`
+
+       **Parameters**:
+
+       - multi_length: number of expected values for a multiple variable
+       - multi_min_length: maximum number of expected values for a multiple variable
+       - multi_max_length: minimum number of expected values for a minimum variable
    * - **unique**
 
        `boolean`