Improve RS documentation.

bug 8570568
Change-Id: I465fcc8ec3e2cc892dc3cb0067aacf5224338102
diff --git a/graphics/java/android/renderscript/AllocationAdapter.java b/graphics/java/android/renderscript/AllocationAdapter.java
index 85d86e5..a6645bb 100644
--- a/graphics/java/android/renderscript/AllocationAdapter.java
+++ b/graphics/java/android/renderscript/AllocationAdapter.java
@@ -22,6 +22,7 @@
 import android.util.TypedValue;
 
 /**
+ * Only intended for use by generated reflected code.
  *
  **/
 public class AllocationAdapter extends Allocation {