Documentation: add a usecase for FS_IOC_READ_VERITY_METADATA

Mention another potential usecase for FS_IOC_READ_VERITY_METADATA:
creating filesystem images which contain fs-verity-enabled files,
without having to redo all of the work in userspace.

Signed-off-by: Allison Karlitskaya <allison.karlitskaya@redhat.com>
Link: https://lore.kernel.org/r/20241126084833.70538-1-allison.karlitskaya@redhat.com


Signed-off-by: Eric Biggers <ebiggers@google.com>