fullscreen-shell: Clarify that present requests assign a surface role

Currently, the spec doesn't say explicitly that present requests assign
a surface role. Given that, it can be viewed as the protocol modifies
an already assigned surface role, e.g. xdg-toplevel, and present requests
only act as hints.

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