docs(config): say which retired folder keys RomM still accepts

[gantoine]

rommapp/romm#4850 stops filesystem.roms_folder and filesystem.firmware_folder from always blocking startup. This updates the "were retired" warning in the configuration reference to match.

• A leftover key is ignored with a warning when its value matches the default layout (roms_folder: roms, firmware_folder: bios), which older releases wrote into every config.yml, or when the matching filesystem.structure key is declared.
• Any other value still stops startup and prints the replacement template.
• Under a Structure B default ({platform}/roms/{game}), a leftover firmware_folder maps to {platform}/<folder>, so firmware_folder: bios with no structure.firmware stops startup and asks for firmware: "{platform}/bios".

Depends on rommapp/romm#4850, so merge it after that ships.

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