Add comment about hasher usage for checksums

Follow up from: https://r8-review.git.corp.google.com/c/r8/+/95540

Change-Id: I2f1cdfe3bfc023c0510e1f87d53f58377bc4bc88
diff --git a/src/main/java/com/android/tools/r8/synthesis/SyntheticClassBuilder.java b/src/main/java/com/android/tools/r8/synthesis/SyntheticClassBuilder.java
index adecc39..572b8d1 100644
--- a/src/main/java/com/android/tools/r8/synthesis/SyntheticClassBuilder.java
+++ b/src/main/java/com/android/tools/r8/synthesis/SyntheticClassBuilder.java
@@ -228,6 +228,7 @@
   }
 
   private <S extends StructuralItem<S>> long hashEntries(List<S>... entryLists) {
+    // Use a hasher that is stable across jvm runs.
     HasherWrapper hasherWrapper = HasherWrapper.murmur3128Hasher();
     for (List<S> entryList : entryLists) {
       entryList.stream().sorted().forEach(e -> e.hash(hasherWrapper));