added required missing field for common yaml fmt

lib/ansible/plugins/inventory/constructed.py

@@ -15,6 +15,11 @@ DOCUMENTATION = '''
         - The JInja2 exprpessions are calculated and assigned to the variables
         - Only variables already available from previous inventories or the fact cache can be used for templating.
         - When I(strict) is False, failed expressions will be ignored (assumes vars were missing).
+    options:
+        plugin:
+            description: token that ensures this is a source file for the 'constructed' plugin.
+            required: True
+            choices: ['constructed']
     extends_documentation_fragment:
       - constructed
 '''