Index: Doc/library/os.rst
===================================================================
--- Doc/library/os.rst	(revision 79422)
+++ Doc/library/os.rst	(working copy)
@@ -437,7 +437,12 @@
 is slightly deceptive; on Unix platforms, sockets and pipes are also referenced
 by file descriptors.
 
+The :meth:`~file.fileno` method can be used to obtain the file descriptor
+associated with a file object when required. Note that using the file
+descriptor directly will bypass the file object methods, ignoring aspects such
+as internal buffering of data.
 
+
 .. function:: close(fd)
 
    Close file descriptor *fd*. Availability: Unix, Windows.
@@ -546,6 +551,11 @@
    Truncate the file corresponding to file descriptor *fd*, so that it is at most
    *length* bytes in size. Availability: Unix.
 
+   .. note::
+      This function is intended for low-level I/O and must be applied to a file
+      descriptor as returned by :func:`os.open`.  To truncate a "file object"
+      returned by the built-in function :func:`open` or :func:`fdopen`, use its
+      :meth:`~file.truncate` method.
 
 .. function:: isatty(fd)