Sadly, just linking to the Histogram best practice document, as done
for `histogram_quantile`, would be confusing here because the best
practice document only deals with quantiles in the context of
Histograms and Summaries, which is very different from the context of
the `quantile` aggregator and `quantile_over_time` function, which is
already a source of a lot of confusion.
Thus, I think the least bad solution is to add a short explanation in
this section directly. There isn't even a good resource on the
internet we can link to. A lot of statisticians use φ-quantiles, but
they don't have a generally accepted name for it.
I have added the explanation after the other detailed explanations of
`count_values`, `topk` and `bottomk`. I think that fits quite nicely
into the flow.
Signed-off-by: beorn7 <beorn@grafana.com>
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