Added docs to XHRSettingsObject

Author: jorbascrumps

src/loader/typedefs/XHRSettingsObject.js

@@ -7,6 +7,7 @@
  * @property {string} [user=''] - Optional username for the XHR request.
  * @property {string} [password=''] - Optional password for the XHR request.
  * @property {integer} [timeout=0] - Optional XHR timeout value.
+ * @property {(object|undefined)} [headers] - This value is used to populate the XHR `setRequestHeader` and is undefined by default.
  * @property {(string|undefined)} [header] - This value is used to populate the XHR `setRequestHeader` and is undefined by default.
  * @property {(string|undefined)} [headerValue] - This value is used to populate the XHR `setRequestHeader` and is undefined by default.
  * @property {(string|undefined)} [requestedWith] - This value is used to populate the XHR `setRequestHeader` and is undefined by default.