[libsigcplusplus] docs: slots: auto: Mention why it is bad.

[Murray Cumming <murrayc src gnome org>]

commit a51e91edd07d36642273635a6448ef5ec410f765
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 1 11:47:03 2016 +0200

    docs: slots: auto: Mention why it is bad.

 sigc++/functors/slot_base.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/functors/slot_base.h b/sigc++/functors/slot_base.h
index affbc9d..24787e2 100644
--- a/sigc++/functors/slot_base.h
+++ b/sigc++/functors/slot_base.h
@@ -230,6 +230,10 @@ struct SIGC_API slot_do_unbind
  * auto sl = sigc::mem_fun(someobj, &SomeClass::somemethod); // Not a slot!
  * @endcode
  *
+ * If you don't explicitly use a sigc::slot then the slot could call a method
+ * on an instance after it has been destroyed even if the method is in a class
+ * that derives from sigc::trackable.
+ *
  * @section slots-with-lambdas C++ Lambdas
  *
  * A C++11 lambda expression is a functor (function object). It is automatically