returns %NULL only if the component doesn't recur, otherwise returns either the recurrence description or at least text like "The meeting recurs"
no extra flags, either returns %NULL or the recurrence description, something like "Every 2 weeks..."
either returns %NULL or the recurrence description prefixed with text like "The meeting recurs", forming something like "The meeting recurs every 2 weeks..."
Influences behaviour of e_cal_recur_describe_recurrence().