include/system/memory.h: Clarify address_space_destroy() behaviour

include/system/memory.h: Clarify address_space_destroy() behaviour

address_space_destroy() doesn't actually immediately destroy the AS;
it queues it to be destroyed via RCU. This means you can't g_free()
the memory the AS struct is in until that has happened.

Clarify this in the documentation.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250929144228.1994037-2-peter.maydell@linaro.org
Signed-off-by: Peter Xu <peterx@redhat.com>
(cherry picked from commit 9e7bfda4909cc688dd0327e17985019f08a78d5d)
(Mjt: this is just a comment fix, but it makes subsequent changes to apply c
leanly)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>