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 <m@maowtm.org>
Link: https://lore.kernel.org/r/6050e764c2679cba01715653e5f1f4f17091d8f8.1759103277.git.m@maowtm.org
[mic: Synchronize date]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: micromaomao

Documentation/security/landlock.rst

@@ -7,7 +7,7 @@ Landlock LSM: kernel documentation
 ==================================
 
 :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,
@@ -110,6 +110,12 @@ Filesystem
 .. kernel-doc:: security/landlock/fs.h
     :identifiers:
 
+Process credential
+------------------
+
+.. kernel-doc:: security/landlock/cred.h
+    :identifiers:
+
 Ruleset and domain
 ------------------
 
@@ -128,6 +134,9 @@ 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
 ========================