diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -956,6 +956,12 @@ are always available.  They are listed h
    The *file* argument must be an object with a ``write(string)`` method; if it
    is not present or ``None``, :data:`sys.stdout` will be used.
 
+   The encoding used when writing to :data:`sys.stdout` or :data:`sys.stderr`)
+   is the value of :envvar:`PYTHONIOENCODING` if defined, otherwise the stream
+   encoding will be retrieved with :func:`os.device_encoding` (which usually
+   only works if Python is connected to a tty, not in a pipeline), otherwise a
+   locale encoding will be used if :func:`locale.getpreferredencoding` returns
+   it, or it will default to ASCII.
 
 .. function:: property(fget=None, fset=None, fdel=None, doc=None)