Update documentation on context ivar

Source/ASDisplayNode.h

@@ -97,8 +97,9 @@ AS_EXTERN NSInteger const ASDefaultDrawingPriority;
 @interface ASDisplayNode : NSObject <ASLocking> {
 @public
   /**
-   * A field for clients to use as they please, for example in implementing category methods on
-   * the base class. Example usage can be found in CatDealsCollectionView.
+   * The _context ivar is unused by Texture, but provided to enable advanced clients to make powerful extensions to base class functionality.
+   * For example, _context can be used to implement category methods on ASDisplayNode that add functionality to all node subclass types.
+   * Code demonstrating this technique can be found in the CatDealsCollectionView example.
    */
   void *_context;
 }