landlock: Expand restrict flags example for ABI version 8

Add LANDLOCK_RESTRICT_SELF_TSYNC to the backwards compatibility example
for restrict flags. This introduces completeness, similar to that of
the ruleset attributes example. However, as the new example can impact
enforcement in certain cases, an appropriate warning is also included.

Additionally, I modified the two comments of the example to make them
more consistent with the ruleset attributes example's.

Signed-off-by: Panagiotis "Ivory" Vasilopoulos <git@n0toose.net>
Co-developed-by: Dan Cojocaru <dan@dcdev.ro>
Signed-off-by: Dan Cojocaru <dan@dcdev.ro>
Reviewed-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20260304-landlock-docs-add-tsync-example-v4-1-819a276f05c5@n0toose.net
[mic: Update date, improve comments consistency, fix newline issue]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Documentation/userspace-api/landlock.rst

@@ -8,7 +8,7 @@ Landlock: unprivileged access control
 =====================================
 
 :Author: Mickaël Salaün
-:Date: January 2026
+:Date: March 2026
 
 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
@@ -197,12 +197,27 @@ similar backwards compatibility check is needed for the restrict flags
 
 .. code-block:: c
 
-    __u32 restrict_flags = LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
-    if (abi < 7) {
-        /* Clear logging flags unsupported before ABI 7. */
+    __u32 restrict_flags =
+        LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON |
+        LANDLOCK_RESTRICT_SELF_TSYNC;
+    switch (abi) {
+    case 1 ... 6:
+        /* Removes logging flags for ABI < 7 */
         restrict_flags &= ~(LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF |
                             LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON |
                             LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF);
+        __attribute__((fallthrough));
+    case 7:
+        /*
+         * Removes multithreaded enforcement flag for ABI < 8
+         *
+         * WARNING: Without this flag, calling landlock_restrict_self(2) is
+         * only equivalent if the calling process is single-threaded. Below
+         * ABI v8 (and as of ABI v8, when not using this flag), a Landlock
+         * policy would only be enforced for the calling thread and its
+         * children (and not for all threads, including parents and siblings).
+         */
+        restrict_flags &= ~LANDLOCK_RESTRICT_SELF_TSYNC;
     }
 
 The next step is to restrict the current thread from gaining more privileges