text: Title: DOC: dtype member docstrings are not tested
  Body: ### Issue with current documentation:
  
  Over at #28001 we discovered that `np.dtype.kind` is not being tested via doctests. I think the problem is in doctests itself, where [it only checks certain items in `obj.__dict__`](https://github.com/python/cpython/blob/7900a85019457c14e8c6abac532846bc9f26760d/Lib/doctest.py#L1064):
  - staticmethod, classmethod, property
  - inspect.isroutine, inspect.isclass
  
  In the case at hand, `np.dtype.kind` is a member, so it is not collected for testing.
  
  ### Idea or request for content:
  
  We should find a work-around, as doctest is a part of the python stdlib, so we cannot simply upgrade the version. cc @ev-br 