Improve javadoc in RollbackRuleAttribute regarding nested classes

Closes gh-24682

Co-authored-by: Sam Brannen <sbrannen@pivotal.io>

spring-tx/src/test/java/org/springframework/transaction/interceptor/RollbackRuleTests.java

@@ -88,4 +88,19 @@ public class RollbackRuleTests {
 				new RollbackRuleAttribute((String) null));
 	}
 
+	@Test
+	public void foundEnclosedExceptionWithEnclosingException() {
+		RollbackRuleAttribute rr = new RollbackRuleAttribute(EnclosingException.class);
+		assertThat(rr.getDepth(new EnclosingException.EnclosedException())).isEqualTo(0);
+	}
+
+	@SuppressWarnings("serial")
+	static class EnclosingException extends RuntimeException {
+
+		@SuppressWarnings("serial")
+		static class EnclosedException extends RuntimeException {
+
+		}
+	}
+
 }