twitter-server: Explain that histograms are windowed
twitter-server: Explain that histograms are windowed

Problem

It's not obvious from the docs that histograms are windowed, so
when you're playing around with TwitterServer at home, and notice
that your test request disappeared from your histograms, you may
be confused.  See https://github.com/twitter/twitter-server/issues/33

Solution

Document that histograms are windowed and why they are.

RB_ID=810682

BuoyantIO/twitter-server,twitter/twitter-server,BuoyantIO/twitter-server,BuoyantIO/twitter-server,twitter/twitter-server,BuoyantIO/twitter-server,twitter/twitter-server