In src/display/api.js, add documentation for the |progressCallback| parameter in |PDFJS.getDocument|

src/display/api.js

@@ -191,6 +191,11 @@ PDFJS.maxCanvasPixels = (PDFJS.maxCanvasPixels === undefined ?
  * parameters: function that needs to be called with new password and reason
  * (see {PasswordResponses}).
  *
+ * @param {function} progressCallback is optional. It is used to be able to
+ * monitor the loading progress of the PDF file (necessary to implement e.g.
+ * a loading bar). The callback receives an {Object} with the properties:
+ * {number} loaded and {number} total.
+ *
  * @return {Promise} A promise that is resolved with {@link PDFDocumentProxy}
  *   object.
  */