DOC: Ensure that we add documentation also as to the dict for types (#18775)
DOC: Ensure that we add documentation also as to the dict for types (#18775)

* DOC: Ensure that we add documentation also as to the dict for types

This ensures that `help(np.dtype)` produces a result.  I am not
exactly sure why it picks up `__doc__` from the dict instead of
`tp_doc` right now. It probably is due to the combination of
inheritance and the fact that the dict always includes `None`
and gets preference during inheritance.
(That probably makes a lot of sense to not inherit the `type`
docstring by default.)

Modifying the dictionary directly is not really good style, either,
but hopefully works.

Closes gh-18740

mhvk/numpy,rgommers/numpy,seberg/numpy,rgommers/numpy,jakirkham/numpy,pdebuyl/numpy,mattip/numpy,mhvk/numpy,charris/numpy,charris/numpy,mattip/numpy,anntzer/numpy,numpy/numpy,endolith/numpy,mattip/numpy,mhvk/numpy,anntzer/numpy,charris/numpy,seberg/numpy,mhvk/numpy,anntzer/numpy,jakirkham/numpy,seberg/numpy,endolith/numpy,jakirkham/numpy,anntzer/numpy,endolith/numpy,numpy/numpy,numpy/numpy,jakirkham/numpy,rgommers/numpy,endolith/numpy,charris/numpy,mattip/numpy,jakirkham/numpy,pdebuyl/numpy,pdebuyl/numpy,seberg/numpy,pdebuyl/numpy,mhvk/numpy,numpy/numpy,rgommers/numpy