Add a recommendation to the timingFunction deprecation message.

Reviewers: O4 Material Motion Apple platform reviewers, O2 Material Motion, markwei

Reviewed By: O4 Material Motion Apple platform reviewers, O2 Material Motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D1888

src/Tween.swift

@@ -96,7 +96,7 @@ public final class Tween: NSObject, Plan {
 
    A nil timing function indicates linear pacing.
    */
-  @available(*, deprecated, message: "No replacement API. Deprecated in v1.2.0.")
+  @available(*, deprecated, message: "Use timingFunction = [timingFunction] instead. Deprecated in v1.2.0.")
   public var timingFunction: CAMediaTimingFunction?
 
   /** Initializes a tween instance with its required properties. */