Document that server-side decorations may always be drawn

There are some compositors that must draw server-side decorations no matter what. If a window is tiled, the server-side decorations are just the borders between tiles, which must be drawn anyway. Qubes OS will eventually support Wayland, and it will always draw server-side decorations, as not doing so would be a security vulnerability.

Instead of declaring such compositors non-conforming, document that compositors are free to draw server-side decorations even if the client does not use xdg-decoration. As a corollary, clients should use the xdg-decoration protocol, as duplicate decorations may result otherwise.

This change is documentation-only. In some cases (such as nested compositors) it might be useful for a client to know that server-side decorations are mandatory. However, I’m not sure how best to express this.