rtnetlink: Fix kdoc of rtnl_af_register().

Commit 26eebdc4 ("rtnetlink: Return int from rtnl_af_register().")
made rtnl_af_register() return int again, and kdoc needs to be fixed up.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241022210320.86111-1-kuniyu@amazon.com


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