Add code comment

Author: luigidellaquila

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/blockhash/BytesRef2BlockHash.java

@@ -146,6 +146,8 @@ public Block[] getKeys() {
             try (BytesRefBlock.Builder b1 = blockFactory.newBytesRefBlockBuilder(positions)) {
                 for (int i = 0; i < positions; i++) {
                     int k1 = (int) (finalHash.get(i) & 0xffffffffL);
+                    // k1 is always positive, it's how hash values are generated, see BytesRefBlockHash.
+                    // For now, we only manage at most 2^31 hash entries
                     if (k1 == 0) {
                         b1.appendNull();
                     } else {