docs: netlink: document struct support for genetlink-legacy

Structures

----------

Legacy families can define C structures both to be used as the contents

of an attribute and as a fixed message header. The plan is to define

the structs in ``definitions`` and link the appropriate attrs.

Legacy families can define C structures both to be used as the contents of

an attribute and as a fixed message header. Structures are defined in

``definitions``  and referenced in operations or attributes. Note that

structures defined in YAML are implicitly packed according to C

conventions. For example, the following struct is 4 bytes, not 6 bytes:

.. code-block:: c

  struct {

          u8 a;

          u16 b;

          u8 c;

  }

Any padding must be explicitly added and C-like languages should infer the

need for explicit padding from whether the members are naturally aligned.

Here is the struct definition from above, declared in YAML:

.. code-block:: yaml

  definitions:

    -

      name: message-header

      type: struct

      members:

        -

          name: a

          type: u8

        -

          name: b

          type: u16

        -

          name: c

          type: u8

Fixed Headers

~~~~~~~~~~~~~

Fixed message headers can be added to operations using ``fixed-header``.

The default ``fixed-header`` can be set in ``operations`` and it can be set

or overridden for each operation.

.. code-block:: yaml

  operations:

    fixed-header: message-header

    list:

      -

        name: get

        fixed-header: custom-header

        attribute-set: message-attrs

Attributes

~~~~~~~~~~

A ``binary`` attribute can be interpreted as a C structure using a

``struct`` property with the name of the structure definition. The

``struct`` property implies ``sub-type: struct`` so it is not necessary to

specify a sub-type.

.. code-block:: yaml

  attribute-sets:

    -

      name: stats-attrs

      attributes:

        -

          name: stats

          type: binary

          struct: vport-stats

Multi-message DO

----------------