diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 1bc9126..eb9cb19 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -677,6 +677,16 @@
    Availability: Unix, Windows.
 
 
+.. data:: SEEK_SET
+          SEEK_CUR
+          SEEK_END
+
+   Parameters to the :func:`lseek` function. Their values are 0, 1, and 2,
+   respectively. Availability: Windows, Unix.
+
+   .. versionadded:: 2.5
+
+
 .. function:: open(file, flags[, mode])
 
    Open the file *file* and set various flags according to *flags* and possibly its
@@ -686,7 +696,8 @@
 
    For a description of the flag and mode values, see the C run-time documentation;
    flag constants (like :const:`O_RDONLY` and :const:`O_WRONLY`) are defined in
-   this module too (see below).
+   this module too (see :ref:`open-constants`).  In particular, on Windows adding
+   :const:`O_BINARY` is needed to open files in binary mode.
 
    Availability: Unix, Windows.
 
@@ -774,6 +785,12 @@
       :func:`fdopen`, or :data:`sys.stdout` or :data:`sys.stderr`, use its
       :meth:`~file.write` method.
 
+
+.. _open-constants:
+
+``open()`` flag constants
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
 The following constants are options for the *flags* parameter to the
 :func:`~os.open` function.  They can be combined using the bitwise OR operator
 ``|``.  Some of them are not available on all platforms.  For descriptions of
@@ -825,16 +842,6 @@
    the C library.
 
 
-.. data:: SEEK_SET
-          SEEK_CUR
-          SEEK_END
-
-   Parameters to the :func:`lseek` function. Their values are 0, 1, and 2,
-   respectively. Availability: Windows, Unix.
-
-   .. versionadded:: 2.5
-
-
 .. _os-file-dir:
 
 Files and Directories
diff --git a/Doc/library/socket.rst b/Doc/library/socket.rst
index 51f3b82..22579ae 100644
--- a/Doc/library/socket.rst
+++ b/Doc/library/socket.rst
@@ -664,10 +664,10 @@
 .. method:: socket.recv_into(buffer[, nbytes[, flags]])
 
    Receive up to *nbytes* bytes from the socket, storing the data into a buffer
-   rather than creating a new string.     If *nbytes* is not specified (or 0),
-   receive up to the size available in the given buffer. See the Unix manual page
-   :manpage:`recv(2)` for the meaning of the optional argument *flags*; it defaults
-   to zero.
+   rather than creating a new string.  If *nbytes* is not specified (or 0),
+   receive up to the size available in the given buffer.  Returns the number of
+   bytes received.  See the Unix manual page :manpage:`recv(2)` for the meaning
+   of the optional argument *flags*; it defaults to zero.
 
    .. versionadded:: 2.5
 
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 5189f1f..8c16d4e 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1757,6 +1757,48 @@
 locking/synchronization.
 
 
+Customizing instance and subclass checks
+----------------------------------------
+
+.. versionadded:: 2.6
+
+The following methods are used to override the default behavior of the
+:func:`isinstance` and :func:`issubclass` built-in functions.
+
+In particular, the metaclass :class:`abc.ABCMeta` implements these methods in
+order to allow the addition of Abstract Base Classes (ABCs) as "virtual base
+classes" to any class or type (including built-in types), and including to other
+ABCs.
+
+.. method:: class.__instancecheck__(self, instance)
+
+   Return true if *instance* should be considered a (direct or indirect)
+   instance of *class*. If defined, called to implement ``isinstance(instance,
+   class)``.
+
+
+.. method:: class.__subclasscheck__(self, subclass)
+
+   Return true if *subclass* should be considered a (direct or indirect)
+   subclass of *class*.  If defined, called to implement ``issubclass(subclass,
+   class)``.
+
+
+Note that these methods are looked up on the type (metaclass) of a class.  They
+cannot be defined as class methods in the actual class.  This is consistent with
+the lookup of special methods that are called on instances, only that in this
+case the instance is itself a class.
+
+.. seealso::
+
+   :pep:`3119` - Introducing Abstract Base Classes
+      Includes the specification for customizing :func:`isinstance` and
+      :func:`issubclass` behavior through :meth:`__instancecheck__` and
+      :meth:`__subclasscheck__`, with motivation for this functionality in the
+      context of adding Abstract Base Classes (see the :mod:`abc` module) to the
+      language.
+
+
 .. _callable-types:
 
 Emulating callable objects