Document the purpose of the test

Author: timtebeek

rewrite-core/src/test/java/org/openrewrite/table/SourceFileResultsTest.java

@@ -57,6 +57,10 @@ void hierarchical() {
 
     @Test
     void deepHierarchyRecordsAllLevels() {
+        // Our Sankey visualization of recipe execution relies on the data table recording all levels of the hierarchy,
+        // not just the leaf recipes. This test ensures that the data table includes rows for both the parent and leaf
+        // recipes, even when the parent recipe doesn't directly perform any transformations (i.e., it has zero
+        // estimated time saving).
         rewriteRun(
           spec -> spec
             .recipeFromYaml(