landlock: Document LANDLOCK_RESTRICT_SELF_TSYNC

landlock: Document LANDLOCK_RESTRICT_SELF_TSYNC

Add documentation for LANDLOCK_RESTRICT_SELF_TSYNC.  It does not need to go
into the main example, but it has a section in the ABI compatibility notes.
In the HTML rendering, the main reference is the system call documentation,
which is included from the landlock.h header file.

Cc: Andrew G. Morgan <morgan@kernel.org>
Cc: John Johansen <john.johansen@canonical.com>
Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20251127115136.3064948-4-gnoack@google.com
[mic: Update date]
Signed-off-by: Mickaël Salaün <mic@digikod.net>