Improve comparison to string.Template and %-formatting.

pep-0498.txt

@@ -86,7 +86,7 @@ PEP-3101 for a detailed rationale. This PEP reuses much of the
 str.format() syntax and machinery, in order to provide continuity with
 an existing Python string formatting mechanism.
 
-However, str.format() is not without its issues. Chief among them are
+However, str.format() is not without its issues. Chief among them is
 its verbosity. For example, the text 'value' is repeated here::
 
   >>> value = 4 * 20
@@ -108,9 +108,24 @@ With an f-string, this becomes::
 f-strings provide a concise, readable way to include expressions
 inside strings.
 
-string.Template has similar shortcomings to str.format(), but also
-supports fewer formatting options. In particular, it does not support
-__format__.
+In this sense, string.Template and %-formatting have similar
+shortcomings to str.format(), but also support fewer formatting
+options. In particular, they do not support __format__, so that there
+is no way to control how a specific object is converted to a string,
+nor can it be extended to additional types that want to control how
+they are converted to strings (such as Decimal and datetime). This
+example is not possible with string.Template::
+
+  >>> value = 1234
+  >>> f'input={value:#0.6x}'
+  'input=0x04d2'
+
+And neither %-formatting nor string.Template can control formatting
+such as::
+
+  >>> date = datetime.date(1991, 10, 12)
+  >>> f'{date} was on a {date:%A}'
+  '1991-10-12 was on a Saturday'
 
 No use of globals() or locals()
 -------------------------------