Mention restriction of the existing pprint module. Add a section on !p for f-strings and str.format().

Author: ericvsmith

peps/pep-0813.rst

@@ -33,7 +33,8 @@ Rationale
 =========
 
 Pretty printing is very useful for displaying complex data structures, like dictionaries read from JSON
-content.  By providing a way for classes to customize how their instances participate in pretty printing,
+content.  However, the existing :mod:`pprint` module can only format builtin objects that it knows about.
+By providing a way for classes to customize how their instances participate in pretty printing,
 users have more options for visually improving the display of their complex data, especially for debugging.
 
 By extending the built-in :func:`print` function to automatically pretty print its output, debugging with
@@ -97,6 +98,14 @@ Built-in :func:`print` takes a new optional argument, appended to the end of the
   performs implicit instantiation.
 
 
+Additions to ``f-strings`` and ``str.format()``
+-----------------------------------------------
+
+In addition to the existing ``!s``, ``!r``, and ``!a`` conversion specifiers, an additional ``!p``
+conversion will be added.  The effect of this specifier with an expression ``value`` will be to call
+:py:meth:`pprint.pformat(value)`. In this initial specification, it will be an error to provide any format
+specifier if ``!p`` is used.
+
 Examples
 ========