[libsigcplusplus] visit_each_type(): Add comment about how it is used.

[Murray Cumming <murrayc src gnome org>]

commit f0e4b353405d07f54f768105429a49d0ca72f2d9
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 15 10:11:38 2016 +0200

    visit_each_type(): Add comment about how it is used.

 sigc++/visit_each.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/visit_each.h b/sigc++/visit_each.h
index e6e9f8f..d2d69c6 100644
--- a/sigc++/visit_each.h
+++ b/sigc++/visit_each.h
@@ -192,6 +192,10 @@ visit_each(const T_action& _A_action, const T_functor& _A_functor)
 /** This function performs a functor on each of the targets
  * of a functor limited to a restricted type.
  *
+ * It is currently used only to call slot_do_bind and slot_do_unbind
+ * only on trackable-derived functors, like a compile-time version of
+ * if(dynamic_cast<trackable*)(&functor) { slot_do_unbind(functor); }
+ *
  * @ingroup sigcfunctors
  */
 template <class T_type, class T_action, class T_functor>