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

Also add to comment in the C code.

1 files changed, 12 insertions, 4 deletions

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 9b12045..592bc48 100644
--- a/Doc/library/io.rst
+++ b/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::