KVM: arm64: Document the protected VM userspace API

EL2 owns a protected vCPU's state once it has run, and the ioctls that
would reach it fail rather than operate on the host copy, which holds
the boot state the VMM wrote plus what the exit handlers copy out.
Describe the resulting API in pkvm.rst, point api.rst at it from each
ioctl that behaves differently, and say beside the PVTIME attribute
in devices/vcpu.rst and the PSCI version register in
fw-pseudo-registers.rst how protected VMs differ. The overview also
states the GIC support pKVM enforces: GICv2 is not supported in
protected mode (vgic-v2.c) and GICv5-based guests aren't registered
under pKVM (vgic-v5.c).

Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20261001135711.1640520-19-fuad.tabba@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>

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