Update GroupByNode documentation.

As highlighted in [here](https://community.influxdata.com/t/kapacitor-tags-in-groupby-node/1330/3)

Quote:

> Kapacitor preserves the tags per point independent of the group by dimensions

However the current v1.3 documentation states that;

> Only tags that are dimensions in the grouping will be preserved; all other tags are dropped.

This is incorrect.

Locate the v1.3 GroupByNode documentation and compare its statement about preserved tags with the linked community quote. Done when the documentation no longer says that non-grouping tags are dropped and accurately reflects the described behavior.

Written by the indexing model from the issue text.