Add dispose API docs to raster source (#10856)

src/ol/source/Raster.js

@@ -801,6 +801,13 @@ class RasterSource extends ImageSource {
   }
 }
 
+/**
+ * Clean up and unregister the worker.
+ * @function
+ * @api
+ */
+RasterSource.prototype.dispose;
+
 /**
  * A reusable canvas context.
  * @type {CanvasRenderingContext2D}