Added a note that the str.format() parser is not suitable for use when implementing f-strings.

pep-0498.txt

@@ -470,6 +470,15 @@ use variables as index values::
   >>> f'a={d[a]}'
   'a=20'
 
+Furthermore, the limited expressions that str.format() understands
+need not be valid Python expressions. For example::
+
+  >>> '{i[";]}'.format(i={'";':4})
+  '4'
+
+For this reason, the str.format() "expression parser" is not suitable
+for use when implementing f-strings.
+
 See [#]_ for a further discussion. It was this observation that led to
 full Python expressions being supported in f-strings.