How to add help text to tabs in admin/structure/types/manage/{node-type}
The summaries are dynamic so they're controlled by JS. It should be something as simple as this:

(function($) {
  Drupal.behaviors.csmFieldsetSummaries = {
    attach: function (context) {
      $('fieldset.csm-form', context).drupalSetSummary(function (context) {
        // Make a decision about what summary to display based on your tab's current value.
        var summary = $('input.foo', context).is(':checked') ? 'Input checked' : 'Input not checked';      

        // Or static strings are fine too.
        summary = 'Static summary';

        return Drupal.t(summary);
      });

      $('fieldset.another-csm-form', context).drupalSetSummary(function (context) {
        // ...
      });
    }
  };
})(jQuery);