[libsigcplusplus] docs: slots: Mention automatic disconnection.

[Murray Cumming <murrayc src gnome org>]

commit 645609e60cba1b7edd3e83e107c7db61482e03db
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 1 11:56:06 2016 +0200

    docs: slots: Mention automatic disconnection.

 sigc++/functors/slot_base.h |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/functors/slot_base.h b/sigc++/functors/slot_base.h
index 24787e2..562005b 100644
--- a/sigc++/functors/slot_base.h
+++ b/sigc++/functors/slot_base.h
@@ -218,6 +218,11 @@ struct SIGC_API slot_do_unbind
  *
  * You can also pass slots as method parameters where you might normally pass a function pointer.
  *
+ * @section slots-auto-disconnect Member Methods and Automatic Disconnection.
+ *
+ * See @ref mem_fun "sigc::mem_fun()" about deriving from sigc::trackable to prevent member
+ * methods from being called after the instance has been destroyed.
+ *
  * @section slots-auto auto
  *
  * sigc::mem_fun() and sigc::ptr_fun() return functors, but those functors are