MINOR: Add FetchSnapshot API doc in KafkaRaftClient

raft/src/main/java/org/apache/kafka/raft/KafkaRaftClient.java

@@ -126,9 +126,16 @@
  *    gracefully resign from the current epoch. This causes remaining voters to immediately
  *    begin a new election.
  *
- * 4) {@link FetchRequestData}: This is the same as the usual Fetch API in Kafka, but we piggyback
- *    some additional metadata on responses (i.e. current leader and epoch). Unlike partition replication,
- *    we also piggyback truncation detection on this API rather than through a separate truncation state.
+ * 4) {@link FetchRequestData}: This is the same as the usual Fetch API in Kafka, but we add snapshot
+ *    check before responding, and we also piggyback some additional metadata on responses (i.e. current
+ *    leader and epoch). Unlike partition replication, we also piggyback truncation detection on this API
+ *    rather than through a separate truncation state.
+ *
+ * 5) {@link FetchSnapshotRequestData}: Sent by the follower to the epoch leader in order to fetch a snapshot.
+ *    This happens when a FetchResponse includes a snapshot ID due to the follower's log end offset being less
+ *    than the leader's log start offset. This API is similar to the Fetch API since the snapshot is stored
+ *    as FileRecords, but we use {@link UnalignedRecords} in FetchSnapshotResponse because the records
+ *    are not necessarily offset-aligned.
  *
  */
 public class KafkaRaftClient<T> implements RaftClient<T> {
@@ -1359,7 +1366,7 @@ private boolean handleFetchSnapshotResponse(
             } else {
                 throw new IllegalStateException(
                     String.format(
-                        "Full log trunctation expected but didn't happen. Snapshot of %s, log end offset %s, last fetched %s",
+                        "Full log truncation expected but didn't happen. Snapshot of %s, log end offset %s, last fetched %s",
                         snapshot.snapshotId(),
                         log.endOffset(),
                         log.lastFetchedEpoch()

raft/src/main/java/org/apache/kafka/raft/NetworkChannel.java

@@ -30,10 +30,9 @@ public interface NetworkChannel extends Closeable {
     int newCorrelationId();
 
     /**
-     * Send an outbound message. This could be either an outbound request
-     * (i.e. an instance of {@link org.apache.kafka.raft.RaftRequest.Outbound})
-     * or a response to a request that was received through {@link #receive(long)}
-     * (i.e. an instance of {@link org.apache.kafka.raft.RaftResponse.Outbound}).
+     * Send an outbound request message.
+     *
+     * @param request outbound request to send
      */
     void send(RaftRequest.Outbound request);
 

raft/src/main/java/org/apache/kafka/raft/ReplicatedLog.java

@@ -264,7 +264,7 @@ default long truncateToEndOffset(OffsetAndEpoch endOffset) {
     Optional<OffsetAndEpoch> oldestSnapshotId();
 
     /**
-     * Notifies the replicted log when a new snapshot is available.
+     * Notifies the replicated log when a new snapshot is available.
      */
     void onSnapshotFrozen(OffsetAndEpoch snapshotId);