Improve jsdoc of util.getObjectProperty to clarify thrown error

See #2703

packages/node_modules/@node-red/util/lib/util.js

@@ -318,9 +318,23 @@ function getMessageProperty(msg,expr) {
 /**
  * Gets a property of an object.
  *
+ * Given the object:
+ *
+ *     {
+ *       "pet": {
+ *           "type": "cat"
+ *       }
+ *     }
+ *
+ * - `pet.type` will return `"cat"`.
+ * - `pet.name` will return `undefined`
+ * - `car` will return `undefined`
+ * - `car.type` will throw an Error (as `car` does not exist)
+ *
  * @param  {Object} msg - the object
  * @param  {String} expr - the property expression
  * @return {any} the object property, or undefined if it does not exist
+ * @throws Will throw an error if the *parent* of the property does not exist
  * @memberof @node-red/util_util
  */
 function getObjectProperty(msg,expr) {