Add warnings about calling File::save() repeatedly.

taglib/wavpack/wavpackfile.h

@@ -133,6 +133,11 @@ namespace TagLib {
 
       /*!
        * Saves the file.
+       *
+       * This returns true if the save was successful.
+       *
+       * \warning In the current implementation, it's dangerous to call save()
+       * repeatedly.  At worst it will corrupt the file.
        */
       virtual bool save();