Man pages matching name ssl_ctx_set_tlsext_servername_callback in section 3ssl:

SSL_CTX_set_tlsext_servername_callback.3ssl.en	debian 10 / libssl-doc
SSL_CTX_set_tlsext_servername_callback.3ssl.en	debian 11 / libssl-doc
SSL_CTX_set_tlsext_servername_callback.3ssl.en	debian 12 / libssl-doc