[libsigcplusplus] limit_derived_target: Add some comments.

[Murray Cumming <murrayc src gnome org>]

commit dcd493aefedafd27da9861bd5c6456bdb6945c3b
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 15 09:40:57 2016 +0200

    limit_derived_target: Add some comments.

 sigc++/visit_each.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/visit_each.h b/sigc++/visit_each.h
index 41d77d6..09e372f 100644
--- a/sigc++/visit_each.h
+++ b/sigc++/visit_each.h
@@ -42,6 +42,7 @@ struct limit_derived_target
   {
     using T_self = limit_derived_target<T_target, T_action>;
 
+    //Only call action_() if T_Target derives from T_Type.
     with_type<T_type, T_self>::execute_(std::forward<T_type>(_A_type), *this);
   }
 
@@ -88,6 +89,7 @@ struct limit_derived_target<T_target*, T_action>
   {
     using T_self = limit_derived_target<T_target*, T_action>;
 
+    //Only call action_() if T_Target derives from T_Type.
     with_type_pointer<T_type, T_self>::execute_(std::forward<T_type>(_A_type), *this);
   }