Add vex extension to the list, add code comments

Signed-off-by: Martin Gaievski <gaievski@amazon.com>

src/main/java/org/opensearch/knn/index/util/Lucene.java

@@ -77,6 +77,6 @@ public float score(float rawScore, SpaceType spaceType) {
 
     @Override
     public List<String> mmapFileExtensions() {
-        return List.of("vec", "vem");
+        return List.of("vec", "vex");
     }
 }

src/main/java/org/opensearch/knn/plugin/KNNPlugin.java

@@ -342,8 +342,17 @@ public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings sett
         return ImmutableList.of(new SystemIndexDescriptor(MODEL_INDEX_NAME, "Index for storing models used for k-NN indices"));
     }
 
+    /**
+     * Plugin can provide additional node settings, that includes new settings or overrides for existing one from core.
+     *
+     * @return settings that are set by plugin
+     */
     @Override
     public Settings additionalSettings() {
+        // We add engine specific extensions to the core list for HybridFS store type. We read existing values
+        // and append ours because in core setting will be replaced by override.
+        // Values are set as cluster defaults and are used at index creation time. Index specific overrides will take priority over values
+        // that are set here.
         final List<String> engineSettings = Arrays.stream(KNNEngine.values())
             .flatMap(engine -> engine.mmapFileExtensions().stream())
             .collect(Collectors.toList());

src/test/java/org/opensearch/knn/index/util/KNNEngineTests.java

@@ -57,7 +57,7 @@ public void testMmapFileExtensions() {
             .flatMap(engine -> engine.mmapFileExtensions().stream())
             .collect(Collectors.toList());
         assertNotNull(mmapExtensions);
-        final List<String> expectedSettings = List.of("vem", "vec");
+        final List<String> expectedSettings = List.of("vex", "vec");
         assertTrue(expectedSettings.containsAll(mmapExtensions));
         assertTrue(mmapExtensions.containsAll(expectedSettings));
     }

src/test/java/org/opensearch/knn/index/util/LuceneTests.java

@@ -119,7 +119,7 @@ public void testVersion() {
     public void testMmapFileExtensions() {
         final List<String> luceneMmapExtensions = Lucene.INSTANCE.mmapFileExtensions();
         assertNotNull(luceneMmapExtensions);
-        final List<String> expectedSettings = List.of("vem", "vec");
+        final List<String> expectedSettings = List.of("vex", "vec");
         assertTrue(expectedSettings.containsAll(luceneMmapExtensions));
         assertTrue(luceneMmapExtensions.containsAll(expectedSettings));
     }