Update mdns.txt for below API changes:
* mdns_resp_add_netif() no longer has dns_ttl argument
* mdns_resp_add_service() no longer has dns_ttl arguemnt
* mdns_resp_add_service() uses enum mdns_sd_proto for proto argument
* Add missing const qualifier at appropriate places
Signed-off-by: Axel Lin <axel.lin@ingics.com>