doc/netlink: Describe nested structs in netlink raw docs

Add a description and example of nested struct definitions
to the netlink raw documentation.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-12-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/userspace-api/netlink/netlink-raw.rst

@@ -158,3 +158,37 @@ alongside a sub-message selector and also in a top level ``attribute-set``, then
 the selector will be resolved using the value 'closest' to the selector. If the
 value is not present in the message at the same level as defined in the spec
 then this is an error.
+
+Nested struct definitions
+-------------------------
+
+Many raw netlink families such as :doc:`tc<../../networking/netlink_spec/tc>`
+make use of nested struct definitions. The ``netlink-raw`` schema makes it
+possible to embed a struct within a struct definition using the ``struct``
+property. For example, the following struct definition embeds the
+``tc-ratespec`` struct definition for both the ``rate`` and the ``peakrate``
+members of ``struct tc-tbf-qopt``.
+
+.. code-block:: yaml
+
+  -
+    name: tc-tbf-qopt
+    type: struct
+    members:
+      -
+        name: rate
+        type: binary
+        struct: tc-ratespec
+      -
+        name: peakrate
+        type: binary
+        struct: tc-ratespec
+      -
+        name: limit
+        type: u32
+      -
+        name: buffer
+        type: u32
+      -
+        name: mtu
+        type: u32