landlock: Document Landlock's file truncation support

Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial.

Adapt the backwards compatibility example and discussion to remove the
truncation flag where needed.

Point out potential surprising behaviour related to truncate.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-12-gnoack3000@gmail.com


Signed-off-by: Mickaël Salaün <mic@digikod.net>