[RenderScript] Add docs advising users about dstAtop bug for support lib.

Bug: 22047392
Change-Id: I5264e39804ebafd53291c5b1ad5513ffff422942
(cherry picked from commit a706ff0e94206b4691f188a8f2d664f1486b2d60)
diff --git a/v8/renderscript/java/src/android/support/v8/renderscript/ScriptIntrinsicBlend.java b/v8/renderscript/java/src/android/support/v8/renderscript/ScriptIntrinsicBlend.java
index 7b1e59b..eef581e 100644
--- a/v8/renderscript/java/src/android/support/v8/renderscript/ScriptIntrinsicBlend.java
+++ b/v8/renderscript/java/src/android/support/v8/renderscript/ScriptIntrinsicBlend.java
@@ -258,6 +258,8 @@
     /**
      * dst = dst.rgb * src.a + (1.0 - dst.a) * src.rgb
      * dst.a = src.a
+     * Note: Before API 23, the alpha channel was not correctly set.
+     *       Please use with caution when targeting older APIs.
      *
      * @param ain The source buffer
      * @param aout The destination buffer