DOC: Use tp_basicsize, not NPY_SIZEOF_PYARRAYOBJECT, for allocation

As mentioned in #2975, add agreed upon text from #2980 to the release notes.

Closes #2980.
Merge pull request #4413 from bfroehle/patch-1

DOC: Use tp_basicsize, not NPY_SIZEOF_PYARRAYOBJECT, for allocation