Update vcgencmd documentation

[lurch]

The switch to KMS and the removal of "Legacy graphics" in Bookworm means that some of the vcgencmd options no longer work, or work differently. https://www.raspberrypi.com/documentation/computers/os.html#vcgencmd

[github-actions]

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[aallan]

Still needs doing.

[lurch]

Fixed on the bookworm branch.