genetlink: fix kdoc warnings

Address a bunch of kdoc warnings:

include/net/genetlink.h:81: warning: Function parameter or member 'module' not described in 'genl_family'
include/net/genetlink.h:243: warning: expecting prototype for struct genl_info. Prototype was for struct genl_dumpit_info instead
include/net/genetlink.h:419: warning: Function parameter or member 'net' not described in 'genlmsg_unicast'
include/net/genetlink.h:438: warning: expecting prototype for gennlmsg_data(). Prototype was for genlmsg_data() instead
include/net/genetlink.h:244: warning: Function parameter or member 'op' not described in 'genl_dumpit_info'

Link: https://lore.kernel.org/r/20221018231310.1040482-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>