add javadoc

Author: ywelsch

core/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -567,6 +567,9 @@ static Engine.Index prepareIndex(DocumentMapperForType docMapper, SourceToParse
         return new Engine.Index(uid, doc, seqNo, primaryTerm, version, versionType, origin, startTime, autoGeneratedIdTimestamp, isRetry);
     }
 
+    /**
+     * Applies an engine operation to the shard, which can be either an index, delete or noop operation.
+     */
     public Engine.Result applyOperation(Engine.Operation operation) throws IOException {
         return applyOperation(getEngine(), operation);
     }