xdg-shell: Clarify what a toplevel by default includes

xdg-shell assumes that the client provides all parts of a toplevel
window, i.e. things like titlebar, drop shadow. There are already things
here and there implies it, but it could be helpful to spell it out.

This doesn't change any semantics - it's still valid, from the
perspective of the protocol, to create a toplevel without any
decorations, and it always has been, it just means that the semantical
intention is for them to be exactly so.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>