docs: rpi isn't supported in modify kernel options

[GairePravesh]

Please describe the question or issue you're facing with "Modify kernel options - Ubuntu Core documentation".

We do mention the setup is for "GNU GRUB bootloader" but should we expect everyone to know RPI doesn't have grub so this doesn't work for it?

I think a note/disclaimer that this doesn't work for RPI images would be helpful,
for example: https://forum.snapcraft.io/t/set-system-system-kernel-cmdline-not-working/50447/2

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Reported from: https://documentation.ubuntu.com/core/how-to-guides/manage-ubuntu-core/modify-kernel-options/

[dilyn-corner]

+1; I was actually just dealing with this with a customer. We should make clear the kernel commandline update feature via a gadget.yaml is only supported for GRUB.