Clarify the Apdex calculation.

The Apdex calculation in the histogram documentation does not actually
match the Apdex as it should be calculated, as any error states should
not be classified as satisfactory of tolerable. This can't be achieved
without including the status code in the http latency labels, which it
is not.

content/docs/practices/histograms.md

@@ -63,7 +63,7 @@ a histogram called `http_request_duration_seconds`.
       sum(rate(http_request_duration_seconds_count[5m])) by (job)
 
 
-You can calculate the well-known [Apdex
+You can approximate the well-known [Apdex
 score](http://en.wikipedia.org/wiki/Apdex) in a similar way. Configure
 a bucket with the target request duration as the upper bound and
 another bucket with the tolerated request duration (usually 4 times
@@ -84,6 +84,9 @@ buckets are
 `le="0.3"` bucket is also contained in the `le="1.2"` bucket; dividing it by 2
 corrects for that.
 
+The calculation does not exactly match the traditional Apdex score, as it
+includes errors in the satisified and tolerable parts of the calculation.
+
 ## Quantiles
 
 You can use both summaries and histograms to calculate so-called φ-quantiles,