Add more general summary to vacuumlazy.c

Add more comments at the top of vacuumlazy.c on heap relation vacuuming
implementation.

Previously vacuumlazy.c only had details related to dead TID storage.
This commit adds a more general summary to help future developers
understand the heap relation vacuum design and implementation at a high
level.

Reviewed-by: Alena Rybakina, Robert Haas, Andres Freund, Bilal Yavuz
Discussion: https://postgr.es/m/flat/CAAKRu_ZF_KCzZuOrPrOqjGVe8iRVWEAJSpzMgRQs%3D5-v84cXUg%40mail.gmail.com

src/backend/access/heap/vacuumlazy.c

@@ -3,6 +3,60 @@
  * vacuumlazy.c
  *	  Concurrent ("lazy") vacuuming.
  *
+ * Heap relations are vacuumed in three main phases. In phase I, vacuum scans
+ * relation pages, pruning and freezing tuples and saving dead tuples' TIDs in
+ * a TID store. If that TID store fills up or vacuum finishes scanning the
+ * relation, it progresses to phase II: index vacuuming. Index vacuuming
+ * deletes the dead index entries referenced in the TID store. In phase III,
+ * vacuum scans the blocks of the relation referred to by the TIDs in the TID
+ * store and reaps the corresponding dead items, freeing that space for future
+ * tuples.
+ *
+ * If there are no indexes or index scanning is disabled, phase II may be
+ * skipped. If phase I identified very few dead index entries or if vacuum's
+ * failsafe mechanism has triggered (to avoid transaction ID wraparound),
+ * vacuum may skip phases II and III.
+ *
+ * If the TID store fills up in phase I, vacuum suspends phase I, proceeds to
+ * phases II and II, cleaning up the dead tuples referenced in the current TID
+ * store. This empties the TID store resumes phase I.
+ *
+ * In a way, the phases are more like states in a state machine, but they have
+ * been referred to colloquially as phases for so long that they are referred
+ * to as such here.
+ *
+ * Manually invoked VACUUMs may scan indexes during phase II in parallel. For
+ * more information on this, see the comment at the top of vacuumparallel.c.
+ *
+ * In between phases, vacuum updates the freespace map (every
+ * VACUUM_FSM_EVERY_PAGES).
+ *
+ * After completing all three phases, vacuum may truncate the relation if it
+ * has emptied pages at the end. Finally, vacuum updates relation statistics
+ * in pg_class and the cumulative statistics subsystem.
+ *
+ * Relation Scanning:
+ *
+ * Vacuum scans the heap relation, starting at the beginning and progressing
+ * to the end, skipping pages as permitted by their visibility status, vacuum
+ * options, and various other requirements.
+ *
+ * When page skipping is not disabled, a non-aggressive vacuum may scan pages
+ * that are marked all-visible (and even all-frozen) in the visibility map if
+ * the range of skippable pages is below SKIP_PAGES_THRESHOLD.
+ *
+ * Once vacuum has decided to scan a given block, it must read the block and
+ * obtain a cleanup lock to prune tuples on the page. A non-aggressive vacuum
+ * may choose to skip pruning and freezing if it cannot acquire a cleanup lock
+ * on the buffer right away. In this case, it may miss cleaning up dead tuples
+ * and their associated index entries (though it is free to reap any existing
+ * dead items on the page).
+ *
+ * After pruning and freezing, pages that are newly all-visible and all-frozen
+ * are marked as such in the visibility map.
+ *
+ * Dead TID Storage:
+ *
  * The major space usage for vacuuming is storage for the dead tuple IDs that
  * are to be removed from indexes.  We want to ensure we can vacuum even the
  * very largest relations with finite memory space usage.  To do that, we set