Update system-firmware.md

[esplinr]

Make clear what happens if the firmware updater is run with external devices plugged in.

[esplinr]

Is there anything else I need to do for this contribution to be accepted?

[jacobgkau]

I don't really think this is necessary (do we need to explain what happens at every step if the user doesn't follow the instructions?), but I guess it could help someone recognize what's wrong if they accidentally leave something plugged in and are only thinking about the symptom.

To try and keep these bullet points from getting so unwieldy people skim them over without really reading them, I've separated the tangential information into sub-bullet points.

[jacobgkau]

@ahoneybun You replaced one linter issue with another. Let me take a look at what it needs.

[esplinr]

Thank you for merging. This problem has repeatedly caught me off-guard since I bought my laptop from System76. Hopefully having the hint be explicit will help me remember next time. I agree that it would be much better for the updater to show an error instead of silently restarting. The current behavior is baffling if you don't remember what a successful update should look like.