Documentation: update pagemap with shmem exceptions

Mentioning the current missing information in the pagemap and alternatives
on how to retrieve it, in case someone stumbles upon unexpected behaviour.

Signed-off-by: Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com>
Reviewed-by: Ivan Teterevkov <ivan.teterevkov@nutanix.com>
Reviewed-by: Florian Schmidt <florian.schmidt@nutanix.com>
Reviewed-by: Carl Waldspurger <carl.waldspurger@nutanix.com>
Reviewed-by: Jonathan Davies <jonathan.davies@nutanix.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20210923064618.157046-2-tiberiu.georgescu@nutanix.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>