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)]
 pub struct IntervalSet<I> {
-    // Start, end
+    // Start, end (both inclusive)
     map: SmallVec<[(u32, u32); 4]>,
     domain: usize,
     _data: PhantomData<I>,