[BUG] Fix missing javadocs error for nested generated types (opensearch-project#13199)

Signed-off-by: Andriy Redko <andriy.redko@aiven.io>

Author: reta

doc-tools/missing-doclet/src/main/java/org/opensearch/missingdoclet/MissingDoclet.java

@@ -332,13 +332,20 @@ private void checkComment(Element element) {
 
     // Ignore classes annotated with @Generated and all enclosed elements in them.
     private boolean isGenerated(Element element) {
-        return element
+        final boolean isGenerated = element
             .getAnnotationMirrors()
             .stream()
             .anyMatch(m -> m
                 .getAnnotationType()
                 .toString() /* ClassSymbol.toString() returns class name */
                 .equalsIgnoreCase("javax.annotation.Generated"));
+
+        if (!isGenerated && element.getEnclosingElement() != null) {
+            // check if enclosing element is generated
+            return isGenerated(element.getEnclosingElement());
+        } 
+
+        return isGenerated;
     }
 
     private boolean hasInheritedJavadocs(Element element) {