Add reference to help('FORMATTING') in format() builtin (GH-166)

Python/bltinmodule.c

@@ -578,12 +578,14 @@ format as builtin_format
 
 Return value.__format__(format_spec)
 
-format_spec defaults to the empty string
+format_spec defaults to the empty string.
+See the Format Specification Mini-Language section of help('FORMATTING') for
+details.
 [clinic start generated code]*/
 
 static PyObject *
 builtin_format_impl(PyObject *module, PyObject *value, PyObject *format_spec)
-/*[clinic end generated code: output=2f40bdfa4954b077 input=6325e751a1b29b86]*/
+/*[clinic end generated code: output=2f40bdfa4954b077 input=88339c93ea522b33]*/
 {
     return PyObject_Format(value, format_spec);
 }