Document distribution statistics.

[jmcarp]

Distributions have two kinds of statistics: histogram bucket counts, which can be converted from cumulative values to deltas by subtraction; and streaming estimates (min, max, quantiles, etc.), which can't. Because streaming estimates can't be subtracted, they always reflect values since the start of the series's epoch, and can't be used to examine a specific time range (e.g., the last hour). Users should probably use histogram statistics and not streaming statistics for monitoring purposes.

This patch updates the inline docs to clarify these differences and steer users toward histograms in most use cases.

[jmcarp]

I almost want to make the distribution streaming estimates private for now, or hide them behind a flag, because I think users probably shouldn't be using them for monitoring applications. For now, I'm just proposing a docs change, but we can also talk about hiding these somehow if useful.