GDK W32: Add a comment for clarity

This is a copy of a similar comment in another place, which explains
why WS_EX_TRANSPARENT windows get a special treatment.

https://bugzilla.gnome.org/show_bug.cgi?id=758484

gdk/win32/gdkwindow-win32.c

@@ -1229,6 +1229,9 @@ gdk_win32_window_hide (GdkWindow *window)
   if (GDK_WINDOW_TYPE (window) == GDK_WINDOW_TOPLEVEL)
     ShowOwnedPopups (GDK_WINDOW_HWND (window), FALSE);
 
+  /* Use SetWindowPos to hide transparent windows so automatic redraws
+   * in other windows can be suppressed.
+   */
   if (GetWindowLong (GDK_WINDOW_HWND (window), GWL_EXSTYLE) & WS_EX_TRANSPARENT)
     {
       SetWindowPos (GDK_WINDOW_HWND (window), SWP_NOZORDER_SPECIFIED,