Fix docs

[eddiewebb]

Follow from #1774 , as the terse closer mentioned the docs explicitly say"

"Picamera2 does not use this type of memory,"

But it dont work without it. Thats a bug in docs wording. Full details in closed issue.

[JamesH65]

Picamera2 does not use that memory. However, the video encoder does, which is presumably where the problem is, but that is not part of Picamera2.

[pelwell]

You are conflating two different effects of reducing the gpu_mem value below the default of 32 (MB).

1. The obvious effect is to reduce the amount of memory reserved for the GPU. When using libcamera/picamera2 the firmware makes little use of its memory carve-out.
2. The other effect is to switch to the cut-down version of the firmware, which lacks the codec and ISP support.

In short, just do what the documentation says and remove all gpu_mem=... settings.