IntervalSet: add comment about representation

Author: RalfJung

compiler/rustc_index/src/interval.rs

@@ -17,7 +17,7 @@ mod tests;
 /// first value of the following element.
 #[derive(Debug,Clone)]
 pubstructIntervalSet<I>{
-// Start, end
+// Start, end (both inclusive)
 map:SmallVec<[(u32,u32);2]>,
 domain:usize,
 _data:PhantomData<I>,