Attached documentation patch describes the purpose of bt_page_items()'s ctid field. This has come up enough times in disaster recovery or testing scenarios that I feel it's worth drawing particular attention to.
How much detail on the b-tree internals do we want to put in the pageinspect documentation? I can see that being useful, but should we also explain e.g. that the first item on each (non-rightmost) page is the high key?
How do I know if I am looking at a non-rightmost page?
