xdg-shell: clarify setting the parent to an unmapped toplevel

The description of xdg_toplevel::set_parent request states:

If the parent is unmapped then its children are managed as
though the parent of the now-unmapped parent has become the
parent of this surface. If no parent exists for the now-unmapped
parent then the children are managed as though they have no
parent surface.

Effectively, if a mapped toplevel is a parent of some other toplevels, it stops being their parent on unmap, suggesting that an unmapped toplevel can't be a parent. However, the protocol implicitly allows setting the parent to a toplevel which is unmapped from the start. Whether this is a deliberate design decision or an oversight, I think this should be clarified.