DOC: nditer.nested_iters are not documented #10009

in pull request #9998 nditers can be used as context managers to simplify writebackifcopy semantics resolution. The problem of how to resolve the semantics now percolates to the nditer.nested_iters function. In the discussion on the pull request, a few issues with nested_iters arose:

• how to return a context-managed tuple of iterators
• the function itself is not documented but extensively tested in test_nditers
• the function is 321 lines of c and perhaps could be pure python (ok, I just added that now, it didn't actually come up in the pull request review)

Opening this as a reminder to fix this after the pull request #9998 is resolved