Add some comment for rr testcase

Includes-commit: 7e8b7fd2842bed6031e3027adc5f3113a41c53ab
Replicated-from: cloudflare/pingora#515

Author: xxargonaut

.bleep

@@ -1 +1 @@
-f11c835e139cb0a0468183e2af3b142742794d7b
+159b358b6a92a5610b6663cddc6eb553ad22f895

pingora-load-balancing/src/selection/weighted.rs

@@ -154,12 +154,16 @@ mod test {
         let mut b2 = Backend::new("1.0.0.1:80").unwrap();
         b2.weight = 8; // 8x than the rest
         let b3 = Backend::new("1.0.0.255:80").unwrap();
+        // sorted with: [b2, b3, b1]
+        // weighted: [0, 0, 0, 0, 0, 0, 0, 0, 1, 2]
         let backends = BTreeSet::from_iter([b1.clone(), b2.clone(), b3.clone()]);
         let hash: Arc<Weighted<RoundRobin>> = Arc::new(Weighted::build(&backends));
 
         // same hash iter over
         let mut iter = hash.iter(b"test");
         // first, should be weighted
+        // weighted: [0, 0, 0, 0, 0, 0, 0, 0, 1, 2]
+        //            ^
         assert_eq!(iter.next(), Some(&b2));
         // fallbacks, should be round robin
         assert_eq!(iter.next(), Some(&b3));
@@ -168,6 +172,9 @@ mod test {
         assert_eq!(iter.next(), Some(&b3));
 
         // round robin, ignoring the hash key
+        // index advanced 5 steps
+        // weighted: [0, 0, 0, 0, 0, 0, 0, 0, 1, 2]
+        //                           ^
         let mut iter = hash.iter(b"test1");
         assert_eq!(iter.next(), Some(&b2));
         let mut iter = hash.iter(b"test1");
@@ -179,6 +186,7 @@ mod test {
         let mut iter = hash.iter(b"test1");
         assert_eq!(iter.next(), Some(&b1));
         let mut iter = hash.iter(b"test1");
+        // rounded
         assert_eq!(iter.next(), Some(&b2));
         let mut iter = hash.iter(b"test1");
         assert_eq!(iter.next(), Some(&b2));