[3.13] Docs C API: Clarify what happens when null bytes are passed to PyUnicode_AsUTF8 (GH-127458) (#129080)

Docs C API: Clarify what happens when null bytes are passed to `PyUnicode_AsUTF8` (GH-127458)
(cherry picked from commit e792f4b)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Stan U. <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Tomas R. <tomas.roun8@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 5 people

Doc/c-api/unicode.rst

@@ -1054,6 +1054,15 @@ These are the UTF-8 codec APIs:
 
    As :c:func:`PyUnicode_AsUTF8AndSize`, but does not store the size.
 
+   .. warning::
+
+      This function does not have any special behavior for
+      `null characters <https://en.wikipedia.org/wiki/Null_character>`_ embedded within
+      *unicode*. As a result, strings containing null characters will remain in the returned
+      string, which some C functions might interpret as the end of the string, leading to
+      truncation. If truncation is an issue, it is recommended to use :c:func:`PyUnicode_AsUTF8AndSize`
+      instead.
+
    .. versionadded:: 3.3
 
    .. versionchanged:: 3.7