vacuumlazy.c: prefer the term "cleanup lock".

The term "super-exclusive lock" is an acceptable synonym of "cleanup
lock".  Even still, switching from one term to the other in the same
file is confusing.  Standardize on "cleanup lock" within vacuumlazy.c.

Per a complaint from Andres Freund.

Author: petergeoghegan

src/backend/access/heap/vacuumlazy.c

@@ -1274,8 +1274,8 @@ lazy_scan_heap(LVRelState *vacrel, VacuumParams *params, bool aggressive)
 		 * By here we definitely have enough dead_tuples space for whatever
 		 * LP_DEAD tids are on this page, we have the visibility map page set
 		 * up in case we need to set this page's all_visible/all_frozen bit,
-		 * and we have a super-exclusive lock.  Any tuples on this page are
-		 * now sure to be "counted" by this VACUUM.
+		 * and we have a cleanup lock.  Any tuples on this page are now sure
+		 * to be "counted" by this VACUUM.
 		 *
 		 * One last piece of preamble needs to take place before we can prune:
 		 * we need to consider new and empty pages.
@@ -1524,8 +1524,8 @@ lazy_scan_heap(LVRelState *vacrel, VacuumParams *params, bool aggressive)
 		}
 
 		/*
-		 * Final steps for block: drop super-exclusive lock, record free space
-		 * in the FSM
+		 * Final steps for block: drop cleanup lock, record free space in the
+		 * FSM
 		 */
 		if (prunestate.has_lpdead_items && vacrel->do_index_vacuuming)
 		{
@@ -2377,8 +2377,8 @@ lazy_vacuum_heap_rel(LVRelState *vacrel)
  *	lazy_vacuum_heap_page() -- free page's LP_DEAD items listed in the
  *						  vacrel->dead_tuples array.
  *
- * Caller must have an exclusive buffer lock on the buffer (though a
- * super-exclusive lock is also acceptable).
+ * Caller must have an exclusive buffer lock on the buffer (though a full
+ * cleanup lock is also acceptable).
  *
  * tupindex is the index in vacrel->dead_tuples of the first dead tuple for
  * this page.  We assume the rest follow sequentially.  The return value is