clarify help for --repo option
    
    Make it clear that it accepts a URL, not just a filesystem path, since
    we expect most users will want to pass a URL.
    
    Also clarified some confusing wording in the man page, and added an
    example.
    
    Change-Id: I0828840b18e4e0a22642b0974f26afa590b4f7f7