code: public DateTimeFormatterBuilder appendInstant(int fractionalDigits) {
          if (fractionalDigits < -1 || fractionalDigits > 9) {
              throw new IllegalArgumentException("Invalid fractional digits: " + fractionalDigits);
          }
          appendInternal(new InstantPrinterParser(fractionalDigits));
          return this;
      }
docs: Appends an instant using ISO-8601 to the formatter with control over
  the number of fractional digits.
  <p>
  Instants have a fixed output format, although this method provides some
  control over the fractional digits. They are converted to a date-time
  with a zone-offset of UTC and printed using the standard ISO-8601 format.
  The localized decimal style is not used.
  <p>
  The {@code fractionalDigits} parameter allows the output of the fractional
  second to be controlled. Specifying zero will cause no fractional digits
  to be output. From 1 to 9 will output an increasing number of digits, using
  zero right-padding if necessary. The special value -1 is used to output as
  many digits as necessary to avoid any trailing zeroes.
  <p>
  When parsing in strict mode, the number of parsed digits must match the
  fractional digits. When parsing in lenient mode, any number of fractional
  digits from zero to nine are accepted.
  <p>
  The instant is obtained using {@link ChronoField#INSTANT_SECONDS INSTANT_SECONDS}
  and optionally (@code NANO_OF_SECOND). The value of {@code INSTANT_SECONDS}
  may be outside the maximum range of {@code LocalDateTime}.
  <p>
  The {@linkplain ResolverStyle resolver style} has no effect on instant parsing.
  The end-of-day time of '24:00' is handled as midnight at the start of the following day.
  The leap-second time of '23:59:59' is handled to some degree, see
  {@link DateTimeFormatter#parsedLeapSecond()} for full details.
  <p>
  An alternative to this method is to format/parse the instant as a single
  epoch-seconds value. That is achieved using {@code appendValue(INSTANT_SECONDS)}.
  
  @param fractionalDigits  the number of fractional second digits to format with,
  from 0 to 9, or -1 to use as many digits as necessary
  @return this, for chaining, not null