Update doc for BlockingIOError and its alias in the io module

Doc/library/exceptions.rst

@@ -396,6 +396,15 @@ depending on the system error code.
    Corresponds to :c:data:`errno` ``EAGAIN``, ``EALREADY``,
    ``EWOULDBLOCK`` and ``EINPROGRESS``.
 
+   In addition to those of :exc:`OSError`, :exc:`BlockingIOError` can have
+   one more attribute:
+
+   .. attribute:: characters_written
+
+      An integer containing the number of characters written to the stream
+      before it blocked.  This attribute is available when using the
+      buffered I/O classes from the :mod:`io` module.
+
 .. exception:: ChildProcessError
 
    Raised when an operation on a child process failed.

Doc/library/io.rst

@@ -109,16 +109,8 @@ High-level Module Interface
 
 .. exception:: BlockingIOError
 
-   Error raised when blocking would occur on a non-blocking stream.  It inherits
-   :exc:`IOError`.
-
-   In addition to those of :exc:`IOError`, :exc:`BlockingIOError` has one
-   attribute:
-
-   .. attribute:: characters_written
-
-      An integer containing the number of characters written to the stream
-      before it blocked.
+   This is a compatibility alias for the builtin :exc:`BlockingIOError`
+   exception.
 
 
 .. exception:: UnsupportedOperation