docs: Add _io module reference.

docs/library/_io.rst

+:mod:`_io` -- input/output streams
+==================================
+
+.. module:: _io
+   :synopsis: input/output streams
+
+This module contains additional types of stream (file-like) objects
+and helper functions.
+
+Functions
+---------
+
+.. function:: open(name, mode='r', **kwargs)
+
+    Open a file. Builtin ``open()`` function is alised to this function.
+    All ports (which provide access to file system) are required to support
+    `mode` parameter, but support for other arguments vary by port.
+
+Classes
+-------
+
+.. class:: FileIO(...)
+
+    This is type of a file open in binary mode, e.g. using ``open(name, "rb")``.
+    You should not instantiate this class directly.
+
+.. class:: TextIOWrapper(...)
+
+    This is type of a file open in text mode, e.g. using ``open(name, "rt")``.
+    You should not instantiate this class directly.
+
+.. class:: StringIO([string])
+.. class:: BytesIO([string])
+
+    In-memory file-like objects for input/output. `StringIO` is used for
+    text-mode I/O (similar to a normal file opened with "t" modifier).
+    `BytesIO` is used for binary-mode I/O (similar to a normal file
+    opened with "b" modifier). Initial contents of file-like objects
+    can be specified with `string` parameter (should be normal string
+    for `StringIO` or bytes object for `BytesIO`). All the usual file
+    methods like ``read()``, ``write()``, ``close()`` are available on
+    these objects, and additionally, following method:
+
+    .. method:: getvalue()
+
+        Get the current contents of the underlying buffer which holds data.

docs/library/index.rst

@@ -30,6 +30,7 @@ library.
        cmath.rst
        _collections.rst
        gc.rst
+       _io.rst
        math.rst
        select.rst
        sys.rst
@@ -52,6 +53,7 @@ library.
        cmath.rst
        _collections.rst
        gc.rst
+       _io.rst
        math.rst
        select.rst
        sys.rst
@@ -89,6 +91,7 @@ library.
 
        _collections.rst
        gc.rst
+       _io.rst
        math.rst
        sys.rst
        ubinascii.rst