New hook for decoding message body

doc/plugin.txt

@@ -204,6 +204,7 @@ but may be out of date soon thereafter.  You never know.  ;-)
   message_body                   functions/mime.php              do_hook
   ^ attachment $type0/$type1     functions/mime.php              do_hook
   attachments_bottom             functions/mime.php              hook_func
+  decode_body                    functions/mime.php              hook_func
   generic_header                 functions/page_header.php       do_hook
   menuline                       functions/page_header.php       do_hook
   internal_link                  functions/page_header.php       hook_func