add javadoc

Signed-off-by: Lantao Jin <ltjin@amazon.com>

Author: LantaoJin

core/src/main/java/org/opensearch/sql/calcite/plan/Scannable.java

@@ -8,6 +8,12 @@
 import org.apache.calcite.linq4j.Enumerable;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
+/**
+ * The customized table scan is implemented in OpenSearch module, to invoke this scan() method in
+ * core module, we add this interface. Now the only implementation is CalciteEnumerableIndexScan.
+ * When a RelNode after optimization is a Scannable, we can directly invoke scan() method to get the
+ * result of the scan instead of codegen and compile via Linq4j expression.
+ */
 public interface Scannable {
 
   public Enumerable<@Nullable Object> scan();