powerpc/paravirt: vcpu_is_preempted() commentary

Add comments more clearly documenting that this function determines whether
hypervisor-level preemption of the VM has occurred.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210928214147.312412-2-nathanl@linux.ibm.com