add a comment to clarify

Author: joseph-gio

crates/bevy_ecs/src/schedule/label.rs

@@ -179,6 +179,8 @@ impl Labels {
             let mut set = IndexSet::default();
             let (idx, _) = set.insert_full(val.clone());
             let old = type_map.insert(set);
+            // We already checked that there is no set for type `L`,
+            // so let's avoid generating useless drop code for the "previous" entry.
             std::mem::forget(old);
             idx as u64
         }