Explain the modified the Apdex formula.

This confuses everyone who compares the formula to the one on Wikipedia.
Make the difference explicit, and in the process remind how the buckets
work.

content/docs/practices/histograms.md

@@ -79,6 +79,10 @@ following expression yields the Apdex score for each job over the last
       sum(rate(http_request_duration_seconds_bucket{le="1.2"}[5m])) by (job)
     ) / 2 / sum(rate(http_request_duration_seconds_count[5m])) by (job)
 
+Note that we divide the sum of both buckets. The reason is that the histogram
+buckets are cumulative. The `le="0.3"` bucket is also contained in the `le="1.2"`
+bucket; dividing it by 2 corrects for that.
+
 ## Quantiles
 
 You can use both summaries and histograms to calculate so-called φ-quantiles,