DOC: Add release snippets for iteration changes (numpy#28081)

* DOC: Add release snippets for iteration changes

Adds a snippet (maybe long?) for iteration order changes and a brief
bullet point about the new public function.

* DOC: fixup bullet point formatting

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* Address review comments

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doc/release/upcoming_changes/27883.c_api.rst

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+* `NpyIter_GetTransferFlags` is now available to check if
+  the iterator needs the Python API or if casts may cause floating point
+  errors (FPE).  FPEs can for example be set when casting ``float64(1e300)``
+  to ``float32`` (overflow to infinity) or a NaN to an integer (invalid value).

doc/release/upcoming_changes/27883.change.rst

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+Changes to the main iterator and potential numerical changes
+------------------------------------------------------------
+The main iterator, used in math functions and via ``np.nditer`` from Python
+and ``NpyIter`` in C, now behaves differently for some buffered iterations.
+This means that:
+
+* The buffer size used will often be smaller than the maximum buffer sized
+  allowed by the ``buffersize`` parameter.
+* The "growinner" flag is now honored with buffered reductions when no operand
+  requires buffering.
+
+For ``np.sum()`` such changes in buffersize may slightly change numerical
+results of floating point operations.
+Users who use "growinner" for custom reductions could notice
+changes in precision (for example, in NumPy we removed it from
+``einsum`` to avoid most precision changes and improve precision
+for some 64bit floating point inputs).