Improve the F-strings and format specifier documentation (GH-4931)

Mention that the format-specifier mini language in f-strings
is the same one used by str.format.

Doc/reference/lexical_analysis.rst

@@ -654,10 +654,11 @@ expression or conversion result.  An empty string is passed when the
 format specifier is omitted.  The formatted result is then included in
 the final value of the whole string.
 
-Top-level format specifiers may include nested replacement fields.
-These nested fields may include their own conversion fields and
-:ref:`format specifiers <formatspec>`, but may not include more
-deeply-nested replacement fields.
+Top-level format specifiers may include nested replacement fields. These nested
+fields may include their own conversion fields and :ref:`format specifiers
+<formatspec>`, but may not include more deeply-nested replacement fields. The
+:ref:`format specifier mini-language <formatspec>` is the same as that used by
+the string .format() method.
 
 Formatted string literals may be concatenated, but replacement fields
 cannot be split across literals.