Update comment for SkMallocPixelRef::NewWithProc.

include/core/SkMallocPixelRef.h

Index: include/core/SkMallocPixelRef.h
diff --git a/include/core/SkMallocPixelRef.h b/include/core/SkMallocPixelRef.h
index 259692da64a4ccf77666cf16100defa3ae73845b..4cb187597e57a974024ec8d847614050e4fc1c93 100644
--- a/include/core/SkMallocPixelRef.h
+++ b/include/core/SkMallocPixelRef.h
@@ -50,6 +50,11 @@ public:
      *
      *  This pixelref will ref() the specified colortable (if not NULL).
      *
+     *  If ReleaseProc is NULL, the pixels will never be released. This
+     *  can be useful if the pixels were stack allocated. However, such an
+     *  SkMallocPixelRef must not live beyond its pixels (e.g. by copying
+     *  an SkBitmap pointing to it, or drawing to an SkPicture).
+     *
      *  Returns NULL on failure.
      */
     typedef void (*ReleaseProc)(void* addr, void* context);