update dox



git-svn-id: http://skia.googlecode.com/svn/trunk@1720 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/pdf/SkPDFStream.h b/include/pdf/SkPDFStream.h
index a975ad6..739177f 100644
--- a/include/pdf/SkPDFStream.h
+++ b/include/pdf/SkPDFStream.h
@@ -32,7 +32,8 @@
 class SkPDFStream : public SkPDFDict {
 public:
     /** Create a PDF stream. A Length entry is automatically added to the
-     *  stream dictionary.
+     *  stream dictionary. The stream may be retained (stream->ref() may be
+     *  called) so its contents must not be changed after calling this.
      *  @param stream The data part of the stream.
      */
     explicit SkPDFStream(SkStream* stream);