Added Java doc

Signed-off-by: Bukhtawar Khan <bukhtawa@amazon.com>

server/src/main/java/org/opensearch/index/translog/transfer/TranslogCheckpointTransferSnapshot.java

@@ -79,6 +79,9 @@ public String toString() {
             .toString();
     }
 
+    /**
+     * Builder for {@link TranslogCheckpointTransferSnapshot}
+     */
     public static class Builder {
         private final long primaryTerm;
         private final long generation;