Issue #22413: Document newline effect on StringIO initializer and getvalue

Also add to comment in the C code.

Doc/library/io.rst

@@ -864,10 +864,16 @@ Text I/O
    An in-memory stream for text I/O.  The text buffer is discarded when the
    :meth:`~IOBase.close` method is called.
 
-   The initial value of the buffer (an empty string by default) can be set by
-   providing *initial_value*.  The *newline* argument works like that of
-   :class:`TextIOWrapper`.  The default is to consider only ``\n`` characters
-   as end of lines and to do no newline translation.
+   The initial value of the buffer can be set by providing *initial_value*.
+   If newline translation is enabled, newlines will be encoded as if by
+   :meth:`~TextIOBase.write`.  The stream is positioned at the start of
+   the buffer.
+
+   The *newline* argument works like that of :class:`TextIOWrapper`.
+   The default is to consider only ``\n`` characters as ends of lines and
+   to do no newline translation.  If *newline* is set to ``None``,
+   newlines are written as ``\n`` on all platforms, but universal
+   newline decoding is still performed when reading.
 
    :class:`StringIO` provides this method in addition to those from
    :class:`TextIOBase` and its parents:
@@ -875,6 +881,8 @@ Text I/O
    .. method:: getvalue()
 
       Return a ``str`` containing the entire contents of the buffer.
+      Newlines are decoded as if by :meth:`~TextIOBase.read`, although
+      the stream position is not changed.
 
    Example usage::
 

Modules/_io/_iomodule.h

@@ -52,7 +52,12 @@ extern PyObject *_PyIncrementalNewlineDecoder_decode(
    which can be safely put aside until another search.
 
    NOTE: for performance reasons, `end` must point to a NUL character ('\0').
-   Otherwise, the function will scan further and return garbage. */
+   Otherwise, the function will scan further and return garbage.
+
+   There are three modes, in order of priority:
+   * translated: Only find \n (assume newlines already translated)
+   * universal: Use universal newlines algorithm
+   * Otherwise, the line ending is specified by readnl, a str object */
 extern Py_ssize_t _PyIO_find_line_ending(
     int translated, int universal, PyObject *readnl,
     int kind, char *start, char *end, Py_ssize_t *consumed);