xml.sax.expatreader module isn't documented

# xml.sax.expatreader module isn't documented

The [xml.sax.expatreader](https://github.com/python/cpython/blob/main/Lib/xml/sax/expatreader.py) module isn't documented. 

The two only occurrences are on the What's New and Changelog from the 3.13 version.

![image](https://github.com/python/cpython/assets/169837/697e842b-a5d8-4fab-847a-2f8322a474c3)
