Add ownership dox for SkShader::asACompose.

Supplement the comment added in
http://code.google.com/p/skia/source/detail?r=14716

R=reed@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/288713002

git-svn-id: http://skia.googlecode.com/svn/trunk@14724 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkShader.h

@@ -359,6 +359,8 @@ public:
     /**
      *  If the shader subclass is composed of two shaders, return true, and if rec is not NULL,
      *  fill it out with info about the shader.
+     *
+     *  These are bare pointers; the ownership and reference count are unchanged.
      */
 
     struct ComposeRec {