Issue #25179: Documentation for formatted string literals aka f-strings

Some of the inspiration and wording is taken from the text of PEP 498 by Eric
V. Smith, and the existing str.format() documentation.
diff --git a/Doc/tutorial/inputoutput.rst b/Doc/tutorial/inputoutput.rst
index 5314fed..f2171f4 100644
--- a/Doc/tutorial/inputoutput.rst
+++ b/Doc/tutorial/inputoutput.rst
@@ -25,7 +25,8 @@
 concatenation operations you can create any layout you can imagine.  The
 string type has some methods that perform useful operations for padding
 strings to a given column width; these will be discussed shortly.  The second
-way is to use the :meth:`str.format` method.
+way is to use :ref:`formatted string literals <f-strings>`, or the
+:meth:`str.format` method.
 
 The :mod:`string` module contains a :class:`~string.Template` class which offers
 yet another way to substitute values into strings.