doc/netlink: Update genetlink-legacy documentation

Add documentation for recently added genetlink-legacy schema attributes.
Remove statements about 'work in progress' and 'todo'.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230825122756.7603-4-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>