Add the ipc shutdown event (#2652)

This event is triggered when the connection to the ipc is about to
shutdown because of a user action such as with a `restart` or `exit`
command. The `change` field indicates why the ipc is shutting down. It
can be either "restart" or "exit".

fixes #2318

docs/ipc

@@ -671,6 +671,8 @@ barconfig_update (4)::
 binding (5)::
 	Sent when a configured command binding is triggered with the keyboard or
 	mouse
+shutdown (6)::
+	Sent when the ipc shuts down because of a restart or exit by user command
 
 *Example:*
 --------------------------------------------------------------------
@@ -829,6 +831,20 @@ input_type (string)::
 }
 ---------------------------
 
+=== shutdown event
+
+This event is triggered when the connection to the ipc is about to shutdown
+because of a user action such as a +restart+ or +exit+ command. The +change
+(string)+ field indicates why the ipc is shutting down. It can be either
++"restart"+ or +"exit"+.
+
+*Example:*
+---------------------------
+{
+ "change": "restart"
+}
+---------------------------
+
 == See also (existing libraries)
 
 [[libraries]]