Update the docs to clarify functionality

[willhlaw]

Motivation from #3 (comment).

• Clarify operations to retrieve and backup to disk (backup), retrieve and restore to another firestore (clone), and push from backup to another firestore (restore).
• Add new commands that better match the intended action
• Deprecate current commands

[jeremylorino]

@willhlaw please assign. i'll pick this up

[willhlaw]

@jeremylorino, that would be great. You can go ahead and submit a PR if you'd like. If you're interested in helping out, then I'll add you as a collaborator to the project and then you'll be able to get assigned.