Rename dead_tuples to dead_items in vacuumlazy.c

Attached patch performs polishing within vacuumlazy.c, as follow-up
work to the refactoring work in Postgres 14. This mainly consists of
changing references of dead tuples to dead items, which reflects the
fact that VACUUM no longer deals with TIDs that might point to
remaining heap tuples with storage -- the TIDs in the array must now
strictly point to LP_DEAD stub line pointers that remain in the heap,
following pruning.

I've also simplified header comments, and comments above the main
entry point functions. These comments made much more sense back when
lazy_scan_heap() was simpler, but wasn't yet broken up into smaller,
better-scoped functions.

If there are no objections, I'll move on this soon. It's mostly just
mechanical changes.

--
Peter Geoghegan