Make ol.render.canvas.Immediate available in the docs

src/ol/render.jsdoc

@@ -0,0 +1,3 @@
+/**
+ * @namespace ol.render
+ */

src/ol/render/ivectorcontext.js

@@ -5,6 +5,8 @@ goog.provide('ol.render.IVectorContext');
 
 
 /**
+ * VectorContext interface. Currently implemented by
+ * {@link ol.render.canvas.Immediate}
  * @interface
  * @todo stability experimental
  */