Documentation/kvm: Update cpuid documentation for steal time and pv eoi

Thanks Michael S Tsirkin for rewriting the description and suggestions.

Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>