docs: explain nodeid use in check task

src/flow/src/adapter.rs

@@ -158,6 +158,7 @@ pub struct FlowWorkerManager {
     flow_err_collectors: RwLock<BTreeMap<FlowId, ErrCollector>>,
     src_send_buf_lens: RwLock<BTreeMap<TableId, watch::Receiver<usize>>>,
     tick_manager: FlowTickManager,
+    /// This node id is only available in distributed mode, on standalone mode this is guaranteed to be `None`
     pub node_id: Option<u32>,
     /// Lock for flushing, will be `read` by `handle_inserts` and `write` by `flush_flow`
     ///

src/flow/src/adapter/flownode_impl.rs

@@ -126,8 +126,11 @@ impl FlowDualEngine {
         allow_create: bool,
         allow_drop: bool,
     ) -> Result<(), Error> {
+        // use nodeid to determine if this is standalone/distributed mode, and retrieve all flows in this node(in distributed mode)/or all flows(in standalone mode)
         let nodeid = self.streaming_engine.node_id;
         let should_exists: Vec<_> = if let Some(nodeid) = nodeid {
+            // nodeid is available, so we only need to check flows on this node
+            // which also means we are in distributed mode
             let to_be_recover = self
                 .flow_metadata_manager
                 .flownode_flow_manager()
@@ -139,6 +142,8 @@ impl FlowDualEngine {
                 })?;
             to_be_recover.into_iter().map(|(id, _)| id).collect()
         } else {
+            // nodeid is not available, so we need to check all flows
+            // which also means we are in standalone mode
             let all_catalogs = self
                 .catalog_manager
                 .catalog_names()