Clarify and fix the documentation for IOBase.close()

Doc/library/io.rst

@@ -246,8 +246,10 @@ I/O Base Classes
 
       Flush and close this stream. This method has no effect if the file is
       already closed. Once the file is closed, any operation on the file
-      (e.g. reading or writing) will raise an :exc:`IOError`. The internal
-      file descriptor isn't closed if *closefd* was False.
+      (e.g. reading or writing) will raise an :exc:`ValueError`.
+
+      As a convenience, it is allowed to call this method more than once;
+      only the first call, however, will have an effect.
 
    .. attribute:: closed