diff --git a/Doc/library/io.rst b/Doc/library/io.rst
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -159,6 +159,21 @@
       example, :class:`BufferedIOBase` provides unoptimized implementations of
       :meth:`~IOBase.readinto` and :meth:`~IOBase.readline`.
 
+   .. note::
+
+      In the rest of this section, the interfaces provided by the ABCs
+      are documented as if they were regular classes. However, it
+      should be kept in mind that ABCs cannot be instantiated
+      directly, and that the documentation of an ABC's method thus
+      does not describe how the method is implemented in the ABC, but
+      how derived classes are *expected* to implement it. For example,
+      the documentation for :meth:`IOBase.close` specifies that "This
+      method has no effect if the file is already closed", but should
+      be understood as "any class claiming to be compatible with
+      :class:`IOBase` must ensure that its ``close`` method may be
+      called multiple times and that it will do nothing if the
+      instance is already closed".
+
 At the top of the I/O hierarchy is the abstract base class :class:`IOBase`.  It
 defines the basic interface to a stream.  Note, however, that there is no
 separation between reading and writing to streams; implementations are allowed