Improve f-strings documentation (GH-3604)

Provide additional examples of using format specifiers in f-strings
Added examples for using integer and date format specifiers.

Doc/reference/lexical_analysis.rst

@@ -676,6 +676,12 @@ Some examples of formatted string literals::
    >>> value = decimal.Decimal("12.34567")
    >>> f"result: {value:{width}.{precision}}"  # nested fields
    'result:      12.35'
+   >>> today = datetime(year=2017, month=1, day=27)
+   >>> f"{today:%b %d, %Y}"  # using date format specifier
+   'January 27, 2017'
+   >>> number = 1024
+   >>> f"{number:#0x}"  # using integer presentation type as format specifier
+   '0x400'
 
 A consequence of sharing the same syntax as regular string literals is
 that characters in the replacement fields must not conflict with the