More stuff in lifetimes.md.

docs/lifetimes.md

@@ -32,7 +32,7 @@ Note that control implementations generally don't need to worry about backend-sp
 Windows has no reference counting for window handles. The only situations where a window handle can be destroyed are
 
 - with a call to `DestroyWindow()`, or
-- by `DefWindowProc()` if it receives a `WM_CLOSE` message
+- by `DefWindowProc()` if it receives a `WM_CLOSE` message (when the user clicks the Close button on the titlebar of a toplevel window, for instance)
 
 The destruction process is simple:
 
@@ -53,6 +53,8 @@ What does this mean? In the normal case, you create a widget, add it to a contai
 
 Note that removing a widget from a container does not call `gtk_widget_destroy()`; it merely decrements the reference count of the widget.
 
+As with Windows, a `::delete-event` signal (which is sent when the user clicks a toplevel window's Close button) that is allowed to propagate will result in the window being destroyed.
+
 TODO describe what we need to do
 TODO remove lifetimes.c?