/v2/query needs to be documented for 1.8 release

[timhallinflux]

With the shift from technical preview to supported... we need to expand the HTTP API docs to include the /v2/query API (in addition to the /v2/write API).

There is some documentation here in the Flux for v1 docs: https://docs.influxdata.com/flux/v0.50/guides/executing-queries#influxdb-api

But this needs to be more fully fleshed out here
https://docs.influxdata.com/influxdb/v1.8/tools/api/

Of note, we need to make very clear how to leverage the bucket and token parameters.
I believe the token parameter construction is the same on between /v2/query and /v2/write -- but it is worth confirming this.