doc: minor fix

mnl_attr_ok(): fix return value type
mnl_attr_put_u8(): remove unused param - len
mnl_attr_put_u8_check(): remove unused param - len
mnl_nlmsg_ok(): fix return value type
mnl_nlmsg_batch_stop(): not return batch size, but release it

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
Signed-off-by: Florian Westphal <fw@strlen.de>