docs should state that one of 'version' or 'version callback' is needed in hook_libraries_info()

The docs for hook_libraries_info() state that both 'version' and 'version callback' are optional.

This is technically true, but if neither is defined, the library will never be detected as installed, as this code in libraries_detect() shows:

  if (!isset($library['version'])) {
    // We support both a single parameter, which is an associative array, and an
    // indexed array of multiple parameters.
    if (isset($library['version arguments'][0])) {
      // Add the library as the first argument.
      $library['version'] = call_user_func_array($library['version callback'], array_merge(array($library), $library['version arguments']));
    }
    else {
      $library['version'] = $library['version callback']($library, $library['version arguments']);
    }
    if (empty($library['version'])) {
      $library['error'] = 'not detected';
      $library['error message'] = t('The version of the %library library could not be detected.', array(
        '%library' => $library['name'],
      ));
      return $library;
    }