Documentation: kvm: clarify histogram units

In the case of histogram statistics, the values are always sample
counts; the unit instead applies to the bucket range.  For example,
halt_poll_success_hist is a nanosecond statistic because the buckets are
for 0ns, 1ns, 2-3ns, 4-7ns etc.  There isn't really any other sensible
interpretation, but clarify this anyway in the Documentation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>