Tweak Quick Edit error strings & update docs

- Tweak Quick Edit error/feedback strings for clarity
- Fix comments to reflect Quick Edit error related API changes
- Fix outdated FileSyncManager docs (unrelated to the two above, but someone on IRC was just expressing confusion about this today so I figured I'd update this too)

@redmunds, would you be able to review?
CC @njx in case of feedback about the string changes