Clean up the format-specs-for-f-strings-only language and add a cross reference

Author: warsaw

peps/pep-0813.rst

@@ -101,8 +101,8 @@ returns a string which is used as the f-string substitution value.  Also for f-s
 specifier is fully compatible with the ``obj=`` form, e.g. ``f'{obj=!p:expression}'``.  If no format
 spec is given, as above :py:func:`python:pprint.pformat` will be used.
 
-Note that ``:expression`` format specs are *not* allowed in ``str.format()`` calls, at least for the
-initial implementation of this PEP.
+Note that format specs are *not* allowed in ``str.format()`` calls, at least for the :ref:`initial
+implementation <deferred>` of this PEP.
 
 
 Additions to the C-API
@@ -259,6 +259,8 @@ were used as the return types.  Ultimately we rejected this because we don't wan
 define a new class or add any imports just to return values from this function.
 
 
+.. _deferred:
+
 Deferred Ideas
 ==============