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>

Author: gnoack

Documentation/userspace-api/landlock.rst

@@ -8,7 +8,7 @@ Landlock: unprivileged access control
 =====================================
 
 :Author: Mickaël Salaün
-:Date: March 2025
+:Date: November 2025
 
 The goal of Landlock is to enable restriction of ambient rights (e.g. global
 filesystem or network access) for a set of processes.  Because Landlock
@@ -604,6 +604,14 @@ Landlock audit events with the ``LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF``,
 sys_landlock_restrict_self().  See Documentation/admin-guide/LSM/landlock.rst
 for more details on audit.
 
+Thread synchronization (ABI < 8)
+--------------------------------
+
+Starting with the Landlock ABI version 8, it is now possible to
+enforce Landlock rulesets across all threads of the calling process
+using the ``LANDLOCK_RESTRICT_SELF_TSYNC`` flag passed to
+sys_landlock_restrict_self().
+
 .. _kernel_support:
 
 Kernel support