This was noted in the tracking issue (gh-12028) but didn't have its own issue. I wanted to refer to these docs today and found them missing.

Documentation is needed in two places:

• https://www.numpy.org/devdocs/reference/arrays.classes.html#special-attributes-and-methods
• https://www.numpy.org/devdocs/user/basics.subclassing.html

A bigger-picture issue is that the subclassing doc nor other docs really discuss the issues with subclassing, and when it would be more appropriate to implement a duck array instead.