docs: netlink: document struct support for genetlink-legacy

Describe the genetlink-legacy support for using struct definitions
for fixed headers and for binary attributes.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>