added comment

Author: martijnvg

server/src/test/java/org/elasticsearch/index/mapper/RuntimeFieldSourceProviderOptimizationTests.java

@@ -31,6 +31,10 @@
 import static org.elasticsearch.xcontent.XContentFactory.jsonBuilder;
 import static org.hamcrest.Matchers.equalTo;
 
+/**
+ * Tests that source provider optimization that filters _source based on the same of source only runtime fields kick in.
+ * This is important for synthetic source, otherwise many doc value and stored fields get loaded in the process.
+ */
 public class RuntimeFieldSourceProviderOptimizationTests extends ESSingleNodeTestCase {
 
     public void testWithSourceProviderOptimization() throws IOException {