Update obsolete heap pruning comments.

Add new comments that spell out what VACUUM expects from heap pruning:
pruning must never leave behind DEAD tuples that still have tuple
storage.  This has at least been the case since commit 8523492d, which
established the principle that vacuumlazy.c doesn't have to deal with
DEAD tuples that still have tuple storage directly, except perhaps by
simply retrying pruning (to handle a rare corner case involving
concurrent transaction abort).

In passing, update some references to old symbol names that were missed
by the snapshot scalability work (specifically commit dc7420c2c9).

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