Man pages matching name coap_resource_notify_observers in section 3:

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