Add bullet point to Before You Begin section in system-firmware doc

[nonergodic]

Tried running the firmware update process on my Thelio mega r2 and ran into the issue that my monitor remained black. Eventually realized that the problem was that my screen was only coming on after the x server had started, i.e. the bios apparently doesn't support the display port ports of the GPU but only the HDMI port.

Adding a note to the documentation to hopefully spare others a similar fate (especially when not familiar with their new machine yet which increases general uncertainty).

[ahoneybun]

@leviport this does not sound correct, can you confirm?

[leviport]

It's not something I've encountered on mega-r2 with any of the GPUs we have in the lab. What GPU are you using?

[nonergodic]

I have two RTX4090s in my rig and with a DP cable I get no signal on boot on either of them until the login screen. :man_shrugging: