netlink: specs: conntrack: describe CTA_HELP_INFO

nfnl_cthelper_to_nlattr() seems to output CTA_HELP_INFO but the spec
doesn't have a definition for it, add one.

Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Link: https://patch.msgid.link/20260910003852.2184031-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/conntrack.yaml

@@ -292,6 +292,12 @@ attribute-sets:
         name: help-name
         type: string
         doc: helper name
+      -
+        name: help-info
+        type: binary
+        doc: >-
+          Private data of a user space helper, only present for helpers
+          registered via nfnetlink_cthelper with a non-zero data length.
   -
     name: nat-proto-attrs
     attributes: