gtkwindow: Document further resize with csd

gtkwindow: Document further resize with csd

Applying the client-side decorations in the configure routine greatly
increases the chances of having the right size for the GtkHEaderBar and
border shadows.

Yet, it may be possible that these sizes change at a later point in
time, if for example the GtkHeaderBar grows in height while adding new
controls.

Mention this possible pitfall in the documentation for
gtk_window_resize().

https://bugzilla.gnome.org/show_bug.cgi?id=756618