/usr/share/man/man3/snmp_async_send.3.gz is in libsnmp-dev 5.7.3+dfsg-1.8ubuntu3.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/netsnmp_session_api.3.gz
/usr/share/man/man3/netsnmp_session_api.3.gz