Closes #11633 Clarify print buffering.

Doc/library/functions.rst

@@ -954,7 +954,9 @@ are always available.  They are listed here in alphabetical order.
    *end*.
 
    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.
+   is not present or ``None``, :data:`sys.stdout` will be used. Output buffering
+   is determined by *file*. Use ``sys.stdout.flush()`` to ensure immediate
+   appearance on a screen.
 
 
 .. function:: property(fget=None, fset=None, fdel=None, doc=None)