backport bin() documentation
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 03525ca..78d2ad1 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -112,6 +112,15 @@
.. versionadded:: 2.3
+.. function:: bin(x)
+
+ Convert an integer number to a binary string. The result is a valid Python
+ expression. If *x* is not a Python :class:`int` object, it has to define an
+ :meth:`__index__` method that returns an integer.
+
+ .. versionadded:: 2.6
+
+
.. function:: bool([x])
Convert a value to a Boolean, using the standard truth testing procedure. If