clarify when reserved is required (Azure#22860)

Author: tfitzmac

specification/web/resource-manager/Microsoft.Web/stable/2022-03-01/AppServicePlans.json

@@ -1559,7 +1559,7 @@
               "readOnly": true
             },
             "reserved": {
-              "description": "If Linux app service plan <code>true</code>, <code>false</code> otherwise.",
+              "description": "If Linux app service plan <code>true</code>, <code>false</code> otherwise. This property is required for Linux app service plans.",
               "default": false,
               "type": "boolean",
               "x-ms-mutability": [