aggregateWindow() function explanation

I would really love a better explanation of the custom aggregation function you supply to aggregateWindow(). I understand that you provide it with a table, but you also provide it with a column. Understanding that the data model is the input is really a series of tables, I'm not sure what to do if I want more than one column to be involved in the aggregation. I think what would really help would be a section that better explains these parameters, what they mean if you're writing a custom aggregation function, and how `createEmpty: true` fits into this. I generally get things working, but my confidence would be higher if there were a little more documentation and a little more theory somewhere.

##### Relevant URLs
- https://docs.influxdata.com/influxdb/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/aggregatewindow/
- https://docs.influxdata.com/influxdb/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/
- https://docs.influxdata.com/influxdb/v2.0/query-data/flux/window-aggregate/
- https://github.com/influxdata/flux/blob/master/docs/user_docs/9-minutes-to-flux.md
- https://github.com/influxdata/flux/blob/master/docs/user_docs/flux-by-example.md

##### What products and version are you using?

Flux on both 2.0 OSS (beta) as well as Influxdb 1.8 with flux enabled