Man pages matching name libssh2_agent_userauth in section 3:

libssh2_agent_userauth.3.en	debian 10 / libssh2-1-dev
libssh2_agent_userauth.3.en	debian 11 / libssh2-1-dev
libssh2_agent_userauth.3.en	debian 12 / libssh2-1-dev