docs/firmware.md: clarify that the firmware utility script performs the firmware backup

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I think adding phrasing like "If you choose to flash the Full ROM, the script will now offer to make a firmware backup" to the start of Step 4 will help clarify to the reader that they don't have to run a separate script/process/thing that isn't there.

I certainly got confused myself and thought I had to do the firmware backup separately.

(I'm citing firmware.sh in the scripts repository for this: backup_firmware is called in flash_coreboot if the firmware is stock, but not in flash_rwlegacy)