Fix some unclear terminology for Sync and Backups

[sigrunixia]

To back up your vault, use a dedicated one-way backup tool that pushes data from your device to a backup location without rewriting local data.

If using a multi-device sync setup, choose ONE device as your backup device. This is typically your most used device, or "primary device".

You use Obsidian Sync on your laptop, tablet, phone, and work desktop. You spend most of the time on your vault on your work desktop, some of the time on your laptop, and rarely on the other two devices. The work desktop would be considered your primary device.

This can be confusing to users who think Obsidian Sync can differentiate and prioritize between devices.