diff --git a/src/ol/render/webgl/webglimmediate.js b/src/ol/render/webgl/webglimmediate.js
index 2a54aedcff..8e7fb7ad81 100644
--- a/src/ol/render/webgl/webglimmediate.js
+++ b/src/ol/render/webgl/webglimmediate.js
@@ -93,6 +93,9 @@ ol.render.webgl.Immediate.prototype.flush = function() {
 
 
 /**
+ * Register a function to be called for rendering at a given zIndex.  The
+ * function will be called asynchronously.  The callback will receive a
+ * reference to {@link ol.render.canvas.Immediate} context for drawing.
  * @param {number} zIndex Z index.
  * @param {function(ol.render.webgl.Immediate)} callback Callback.
  * @api