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

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