Improving scp page

In the spirit of making this even more clear to people unfamiliar with the CLI, I attempted to clarify the tl;drs for `scp`.

- Filenames now have an extension to make it especially clear they're not dealing with a directory (I know directories can have extensions and files don't need extensions, but generally this isn't understood as well as it should be)
- Made captions clearer
- Added syntax for copying a file from one host to another, I've needed to do this a few times and used to scp the file down to my local machine and scp it to the other host (the shame!). If you think this isn't a common use case, though, feel free to leave it out, as this file is getting a bit long.