KVM: arm64: Document NV caps and vcpu flags

Describe the two new vcpu flags that control NV, together with
the capabilities that advertise them.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20250514103501.2225951-18-maz@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>