manual missing --keygen description

https://www.torproject.org/docs/tor-manual-dev.html.en

OfflineMasterKey 0|1

    If non-zero, the Tor relay will never generate or load its master secret key. Instead, you’ll have to use "tor --keygen" to manage the master secret key. (Default: 0)

but the manual does not contain a description of the --keygen parameter.