Add javadoc

Author: ioanatia

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/PlannerUtils.java

@@ -69,6 +69,15 @@
 
 public class PlannerUtils {
 
+    /**
+     * When the plan contains children like {@code MergeExec} resulted from the planning of commands such as FORK,
+     * we need to break the plan into sub plans and a main coordinator plan.
+     * The result pages from each sub plan will be funneled to the main coordinator plan.
+     * To achieve this, we wire each sub plan with a {@code ExchangeSinkExec} and add a {@code ExchangeSourceExec}
+     * to the main coordinator plan.
+     * There is an additional split of each sub plan into a data node plan and coordinator plan.
+     * This split is not done here, but as part of {@code PlannerUtils#breakPlanBetweenCoordinatorAndDataNode}.
+     */
     public static Tuple<List<PhysicalPlan>, PhysicalPlan> breakPlanIntoSubPlansAndMainPlan(PhysicalPlan plan) {
         var subplans = new Holder<List<PhysicalPlan>>();
         PhysicalPlan mainPlan = plan.transformUp(MergeExec.class, me -> {