fix(cli): Clarify role of --destination
fix(cli): Clarify role of --destination

Making it clear that it is for the generated-site's output and not any
other sub-command (like `cobalt new`).  Really, it shouldn't exist for
those.

cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs