doc/netlink: Describe nested structs in netlink raw docs

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