xdg-shell: Make sure wording reflects expectations

The wording in xdg-shell's `set_*` requests implies the compositor
*will* honour the client's request.
This would give clients the control over their actual state, while the
general expectation is that clients kindly ask for state changes which
the compositor may follow.
This patch ensures the actual protocol text reflects these expectations.

Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>