operations/external/png-save: Fix the description

Sink operations don't pass the buffer through.

Other file saving operations mention the library that is used to write
the files, and it is generally useful information. So let's add that
too.

operations/external/png-save.c

@@ -228,7 +228,7 @@ gegl_op_class_init (GeglOpClass *klass)
   "title",       _("PNG File Saver"),
   "categories" , "output",
   "description",
-        _("PNG image saver (passes the buffer through, saves as a side-effect.)"),
+        _("PNG image saver, using libpng"),
         NULL);
 
   gegl_extension_handler_register_saver (".png", "gegl:png-save");