[libsigcplusplus] docs: mem_fun: Improve the note about auto-disconnection.

[Murray Cumming <murrayc src gnome org>]

commit ba37fad02cc0ce4245b23ef22712a975c6525be5
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 1 12:04:50 2016 +0200

    docs: mem_fun: Improve the note about auto-disconnection.
    
    Because "cleared" doesn't really tell us how this benefits us.

 sigc++/functors/mem_fun.h |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)
---
diff --git a/sigc++/functors/mem_fun.h b/sigc++/functors/mem_fun.h
index e9dc65f..277ecbb 100644
--- a/sigc++/functors/mem_fun.h
+++ b/sigc++/functors/mem_fun.h
@@ -17,9 +17,11 @@ namespace sigc
  *
  * Optionally, a reference or pointer to an object can be bound to the functor.
  *
- * @note Only if the object type inherits from sigc::trackable, and the
- * functor returned from mem_fun() is assigned to a sigc::slot, is the functor
- * automatically cleared when the object goes out of scope!
+ * @note If the object type inherits from sigc::trackable, and the
+ * functor returned from mem_fun() is assigned to a sigc::slot, the functor
+ * will be automatically cleared when the object goes out of scope. Invoking
+ * that slot will then have no effect and will not try to use the destroyed
+ * instance.
  *
  * If the member function pointer is to an overloaded type, you must specify
  * the types using template arguments starting with the first argument.