Improve setCallbacks doc

Co-authored-by: Brian Sweeney <brian@eclecticgeek.com>

src/Dompdf.php

@@ -1420,12 +1420,20 @@ public function set_callbacks($callbacks)
 
     /**
      * Sets callbacks for events like rendering of pages and elements.
-     * The callbacks array contains arrays with 'event' set to 'begin_page',
-     * 'end_page', 'begin_frame', or 'end_frame' and 'f' set to a function or
-     * object plus method to be called.
      *
-     * The function 'f' must take an array as argument, which contains info
-     * about the event.
+     * The callbacks array should contain arrays with `event` set to a callback
+     * event name and `f` set to a function or object plus method to be called.
+     *
+     * The available callback events are:
+     * * `begin_page_reflow`: called before page reflow
+     * * `begin_frame`: called before a frame is rendered
+     * * `end_frame`: called after frame rendering is complete
+     * * `begin_page_render`: called before a page is rendered
+     * * `end_page_render`: called after page rendering is complete
+     *
+     * The function `f` must take an array as argument, which contains info
+     * about the event (`[0 => Canvas, 1 => Frame, "canvas" => Canvas,
+     * "frame" => Frame]`).
      *
      * @param array $callbacks the set of callbacks to set
      */