gh-145633: Refine notes on non-IEEE platforms (#145845)

* We don't specify what happens on non-IEEE platforms.
* Use rather PY_LITTLE_ENDIAN to get native endianness.
* Mention that unpack functions don't fail in CPython.
* Mention that PyFloat_Pack8 doesn't fail in CPython.

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>