[Javadoc/Markdown] JavaDocRegion child elements are displayed outside the region

Based on the comment from https://github.com/eclipse-jdt/eclipse.jdt.core/pull/5313#issuecomment-5409550800

When a JavaDocRegion is present in a Javadoc comment, its child elements are displayed outside the JavaDocRegion in ASTView. This issue occurs with both traditional Javadoc comments and Markdown Javadoc comments.

The JavaDocRegion has the correct source range, and its child elements also have the correct source ranges. However, the child elements are not shown under the corresponding JavaDocRegion in ASTView, which makes the AST structure confusing.

```
/// {@snippet :
/// // @highlight region substring="println"
/// System.out.println("line 1");
/// System.out.println("line 2");
/// // @end
/// }

```