Add signatures for once.

src/ol/control/FullScreen.js

@@ -32,6 +32,14 @@ const FullScreenEventType = {
   LEAVEFULLSCREEN: 'leavefullscreen',
 };
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes|
+ *     'enterfullscreen'|'leavefullscreen', import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|
+ *     'enterfullscreen'|'leavefullscreen'|import("../ObjectEventType").Types>} FullScreenOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {string} [className='ol-full-screen'] CSS class name.
@@ -81,14 +89,15 @@ class FullScreen extends Control {
     });
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes|
-     *     'enterfullscreen'|'leavefullscreen', import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|
-     *     'enterfullscreen'|'leavefullscreen'|import("../ObjectEventType").Types>}
+     * @type {FullScreenOnSignature}
      */
     this.on;
 
+    /***
+     * @type {FullScreenOnSignature}
+     */
+    this.once;
+
     /**
      * @private
      * @type {string}