DOC: Fix reference warning in Arrayterator and recfunctions.

[liang3zy22]

Fix following reference warning:

numpy/lib/arrayterator.py:docstring of numpy.lib.arrayterator.Arrayterator:34: WARNING: py:obj reference target not found: ndenumerate
numpy/lib/arrayterator.py:docstring of numpy.lib.arrayterator.Arrayterator:36: WARNING: py:obj reference target not found: flatiter
numpy/lib/arrayterator.py:docstring of numpy.lib.arrayterator.Arrayterator:38: WARNING: py:obj reference target not found: memmap
numpy/lib/recfunctions.py:docstring of numpy.lib.recfunctions.structured_to_unstructured:21: WARNING: py:obj reference target not found: np.ndarray
numpy/lib/recfunctions.py:docstring of numpy.lib.recfunctions.structured_to_unstructured:21: WARNING: py:obj reference target not found: np.recarray
numpy/lib/recfunctions.py:docstring of numpy.lib.recfunctions.structured_to_unstructured:21: WARNING: py:obj reference target not found: np.memmap

[rossbar]

The doc build failures appear to be coming from sphinx itself rather than this PR: sphinx-doc/sphinx#11608. Specifically, it looks like we're hitting sphinx-doc/sphinx#11605 which details a problem with the latest sphinx + breathe.

I'd vote to give it a day or so to see if a patch comes out; if not, then we should probably pin sphinx until breathe is updated.

[mattip]

Thanks @liang3zy22