doc: netlink: Fix op pre and post fields in generated .rst

doc: netlink: Fix op pre and post fields in generated .rst

The generated .rst has pre and post headings without any values, e.g.
here:

https://docs.kernel.org/6.9/networking/netlink_spec/dpll.html#device-id-get

Emit keys and values in the generated .rst

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240528140652.9445-5-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>