doc/netlink: Describe sub-message selector resolution

Update the netlink-raw docs to add a description of sub-message selector
resolution to explain that selector resolution is constrained by the
spec.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-4-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>