xdg-shell: Clarify xdg_surface.ack_configure behavior

The current wording is open to interpretation. The updated version clarifies that the client is not required to commit the surface immediately after receiving a configure event (for example it can wait N milliseconds, paint and commit the surface) but it must still do it within a reasonable timeframe.

The main motivation behind this behavior is that some Wayland compositors use configure events to synchronize the server side double buffered state with the client side state. If the client decides that the received configure event can be postponed indefinetely, this can leave the compositor with outdated current state, e.g. wrong position, and so on.

Signed-off-by: Vlad Zahorodnii vlad.zahorodnii@kde.org