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 <[e-mail]>

Documentation/virt/kvm/api.rst

···
     by the ``hist_param`` field. The range of the Nth bucket (1 <= N < ``size``)
     is [``hist_param``*(N-1), ``hist_param``*N), while the range of the last
     bucket is [``hist_param``*(``size``-1), +INF). (+INF means positive infinity
-    value.) The bucket value indicates how many samples fell in the bucket's range.
+    value.)
   * ``KVM_STATS_TYPE_LOG_HIST``
     The statistic is reported as a logarithmic histogram. The number of
     buckets is specified by the ``size`` field. The range of the first bucket is
     [0, 1), while the range of the last bucket is [pow(2, ``size``-2), +INF).
     Otherwise, The Nth bucket (1 < N < ``size``) covers
-    [pow(2, N-2), pow(2, N-1)). The bucket value indicates how many samples fell
-    in the bucket's range.
+    [pow(2, N-2), pow(2, N-1)).
 
 Bits 4-7 of ``flags`` encode the unit:
 
···
     statistics of "peak" type will never go back from 1 to 0.  Boolean
     statistics can be linear histograms (with two buckets) but not logarithmic
     histograms.
+
+Note that, in the case of histograms, the unit applies to the bucket
+ranges, while the bucket value indicates how many samples fell in the
+bucket's range.
 
 Bits 8-11 of ``flags``, together with ``exponent``, encode the scale of the
 unit:
···
 
 The ``bucket_size`` field is used as a parameter for histogram statistics data.
 It is only used by linear histogram statistics data, specifying the size of a
-bucket.
+bucket in the unit expressed by bits 4-11 of ``flags`` together with ``exponent``.
 
 The ``name`` field is the name string of the statistics data. The name string
 starts at the end of ``struct kvm_stats_desc``.  The maximum length including