Man pages matching name coap_new_client_session in section 3:

coap_new_client_session.3.en	debian 10 / libcoap2
coap_new_client_session.3.en	debian 11 / libcoap2
coap_new_client_session.3.en	debian 12 / libcoap3
coap_new_client_session.3.en	debian 13 / libcoap3t64