landlock: Make docs in cred.h and domain.h visible

Currently even though the structures in these files have documentation,
they are not shown in the "Landlock LSM: kernel documentation" page.

Signed-off-by: Tingmao Wang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[mic: Synchronize date]
Signed-off-by: Mickaël Salaün <[e-mail]>

Documentation/security/landlock.rst

···
 ==================================
 
 :Author: Mickaël Salaün
-:Date: March 2025
+:Date: September 2025
 
 Landlock's goal is to create scoped access-control (i.e. sandboxing).  To
 harden a whole system, this feature should be available to any process,
···
 .. kernel-doc:: security/landlock/fs.h
     :identifiers:
 
+Process credential
+------------------
+
+.. kernel-doc:: security/landlock/cred.h
+    :identifiers:
+
 Ruleset and domain
 ------------------
 
···
 makes the reasoning much easier and helps avoid pitfalls.
 
 .. kernel-doc:: security/landlock/ruleset.h
+    :identifiers:
+
+.. kernel-doc:: security/landlock/domain.h
     :identifiers:
 
 Additional documentation