@Override
    protected void onSerialize(Serializer buf) {
        super.onSerialize(buf);
        byte[] utf8 = Utf8.toBytes(docId);
        buf.putInt(null, utf8.length);
        buf.put(null, utf8);
        summary.serialize(buf);
    }
Obtain the local document id of this hit.

@return The local document id.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/VdsHit.java