Clarify the meaning of the 'local' option

The present description of the 'local' option is a little confusing: "Set to true to add the host to the local known_hosts file". It's not necessarily obvious what 'local known_hosts' means. The system-wide `/etc/ssh/ssh_known_hosts` file is just as 'local' to the host the program runs on as is the user-specific `$HOME/.ssh/known_hosts` file.

This pull request explicitly elucidates the meaning of this option, so that other users won't have to go read the step's source code to find out whether they guessed its meaning correctly.