documentation improvement: set operations sort the index #3123
Description
though clear from the examples, an explicit hint that set operations will re-sort your indices in ascending order would be helpful (section 7.3.1 "Set operations on Index objects"). i had indicies like "QK_1 ... QK_9 QK_10" and afterwards they got sorted as "QK_1 QK_10 QK_2...".