add some minor comments to Range

DataStructures/Range.h

@@ -57,6 +57,7 @@ template <typename Integer> class range
     Integer operator*() const { return iter; }
 };
 
+// convenience function to construct an integer range with type deduction
 template <typename Integer> range<Integer> irange(Integer first, Integer last)
 {
     return range<Integer>(first, last);