doc: Clarify 'key' option for host key on serve sftp
doc: Clarify 'key' option for host key on serve sftp

The option --key would set the sftp host key. It could be mistaken for a default-user-key. Instead, explicitly call it 'host key' to avoid confusion.

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone