netlink: specs: devlink: describe DEVLINK_ATTR_NESTED_DEVLINK

The "# TODO: fill in the attributes in between" gap before selftests
(176) hides exactly one attribute, DEVLINK_ATTR_NESTED_DEVLINK (175).
devlink_nl_fill() calls devlink_nl_nested_fill() unconditionally, which
emits one of these per entry in devlink->nested_rels (mlx5, ice).

Unlike the other TODO gaps this is a single attribute of an already
modelled type, so just fill it in.

Link: https://patch.msgid.link/20260915161341.1053476-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/devlink.yaml

@@ -858,13 +858,14 @@ attribute-sets:
         name: linecard-supported-types
         type: nest
         nested-attributes: dl-linecard-supported-types
-
-      # TODO: fill in the attributes in between
-
+      -
+        name: nested-devlink
+        type: nest
+        multi-attr: true
+        nested-attributes: dl-nested-devlink
       -
         name: selftests
         type: nest
-        value: 176
         nested-attributes: dl-selftest-id
       -
         name: rate-tx-priority
@@ -1351,6 +1352,19 @@ attribute-sets:
       -
         name: index
 
+  -
+    name: dl-nested-devlink
+    subset-of: devlink
+    attributes:
+      -
+        name: bus-name
+      -
+        name: dev-name
+      -
+        name: index
+      -
+        name: netns-id
+
 operations:
   enum-model: directional
   list:
@@ -1376,6 +1390,7 @@ operations:
             - index
             - reload-failed
             - dev-stats
+            - nested-devlink
       dump:
         reply: *get-reply