Add comment EarlyDateTest checks taxonomy-specific test

Problem/Motivation

> * Posts an article with a taxonomy term and a date prior to 1970.

Add comment why EarlyDateTest checks taxonomy-specific test

Some old git blame #985292: Nodes published before 1970 won't insert in taxonomy index

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet