docs: netlink: document the sub-type attribute property

Add a definition for sub-type to the protocol spec doc and a description of
its usage for C arrays in genetlink-legacy.

Signed-off-by: Donald Hunter <[e-mail]>
Reviewed-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/userspace-api/netlink/genetlink-legacy.rst

···
           type: binary
           struct: vport-stats
 
+C Arrays
+--------
+
+Legacy families also use ``binary`` attributes to encapsulate C arrays. The
+``sub-type`` is used to identify the type of scalar to extract.
+
+.. code-block:: yaml
+
+  attributes:
+    -
+      name: ports
+      type: binary
+      sub-type: u32
+
 Multi-message DO
 ----------------
 

Documentation/userspace-api/netlink/specs.rst

···
 The validation policy in the kernel is formed by combining the type
 definition (``type`` and ``nested-attributes``) and the ``checks``.
 
+sub-type
+~~~~~~~~
+
+Legacy families have special ways of expressing arrays. ``sub-type`` can be
+used to define the type of array members in case array members are not
+fully defined as attributes (in a bona fide attribute space). For instance
+a C array of u32 values can be specified with ``type: binary`` and
+``sub-type: u32``. Binary types and legacy array formats are described in
+more detail in :doc:`genetlink-legacy`.
+
 operations
 ----------