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 <[email protected]>
Reviewed-by: Ivan Teterevkov <[email protected]>
Reviewed-by: Florian Schmidt <[email protected]>
Reviewed-by: Carl Waldspurger <[email protected]>
Reviewed-by: Jonathan Davies <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

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