KVM: arm64: Document KVM_ARM_SET_CNT_OFFSETS and co

Add some basic documentation on the effects of KVM_ARM_SET_CNT_OFFSETS.

Reviewed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230330174800.2677007-16-maz@kernel.org