mm/memory: document restore_exclusive_pte()

Let's document how this function is to be used, and why the folio lock is
involved.

Link: https://lkml.kernel.org/r/20250226132257.2826043-5-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>