[libsigcplusplus] Add a comment.

[Murray Cumming <murrayc src gnome org>]

commit 579041da74bf0927283d4c581dd7020ef03a87df
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 15 10:40:59 2016 +0200

    Add a comment.

 sigc++/signal.h |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/signal.h b/sigc++/signal.h
index 56de251..7a4fad5 100644
--- a/sigc++/signal.h
+++ b/sigc++/signal.h
@@ -872,6 +872,7 @@ public:
   using slot_type = slot<T_return(T_arg...)>;
   using slot_list_type = slot_list<slot_type>;
   using iterator = typename slot_list_type::iterator;
+  //TODO: Test these? These type aliases are currently unused in libsigc++ or its tests:
   using const_iterator = typename slot_list_type::const_iterator;
   using reverse_iterator = typename slot_list_type::reverse_iterator;
   using const_reverse_iterator = typename slot_list_type::const_reverse_iterator;