-speed: note that XChaCha20 is not selectable
-speed: note that XChaCha20 is not selectable

This is unclear to users, as seen on
https://github.com/rfjakob/gocryptfs/issues/452#issuecomment-828836651

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs