incorrect docs in DateFormatter::format()

Problem/Motivation

    // Fall back to the 'medium' date format type if the format string is
    // empty, either from not finding a requested date format or being given an
    // empty custom format string.
    if (empty($format)) {
      $format = $this->dateFormat('fallback', $langcode)->getPattern();
    }

Comment and code don't agree about which format is being used.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet