It doesn’t say anything about groups OR appending one page to another one, it says marking another one as its parent, and mentions close_page(). close_page() explains what the parent page is for.
Well, that’s one of the uses, the other one is that the new page is placed after the last page added with add_page() to the same parent. Either way it doesn’t particularly matter API-wise, that’s just making clicking 3 links from another page work like it would in any browser. - you’d see:
current page
link 1
link 2
link 3
next page
In this order. And then when you close link 3, it will go back to current page, not to link 2.