add some documentation/notes on inner design

Author: Hendrik Muhs

x-pack/plugin/ml-feature-index-builder/src/main/java/org/elasticsearch/xpack/ml/featureindexbuilder/job/FeatureIndexBuilderIndexer.java

@@ -64,7 +64,12 @@ protected IterationResult<Map<String, Object>> doProcess(SearchResponse searchRe
     }
 
     /*
-     * Parses the result and creates indexable documents
+     * Parses the result and creates a stream of indexable documents
+     *
+     * Implementation decisions:
+     *
+     * Extraction uses generic maps as intermediate exchange format in order to hook in ingest pipelines/processors
+     * in later versions, see {@link IngestDocument).
      */
     private Stream<IndexRequest> processBucketsToIndexRequests(CompositeAggregation agg) {
         String indexName = job.getConfig().getDestinationIndex();