Add comment & docs about no vacuum truncation with sto.

Omission noted by Andres Freund.

doc/src/sgml/config.sgml

@@ -2091,6 +2091,15 @@ include_dir 'conf.d'
          transaction ID wraparound may occur in much shorter time frames.
         </para>
 
+        <para>
+         When this feature is enabled, freed space at the end of a relation
+         cannot be released to the operating system, since that could remove
+         information needed to detect the <literal>snapshot too old</>
+         condition.  All space allocated to a relation remains associated with
+         that relation for reuse only within that relation unless explicitly
+         freed (for example, with <command>VACUUM FULL</>).
+        </para>
+
         <para>
          This setting does not attempt to guarantee that an error will be
          generated under any particular circumstances.  In fact, if the

src/backend/commands/vacuumlazy.c

@@ -1663,6 +1663,15 @@ lazy_cleanup_index(Relation indrel,
  * Don't even think about it unless we have a shot at releasing a goodly
  * number of pages.  Otherwise, the time taken isn't worth it.
  *
+ * Also don't attempt it if we are doing early pruning/vacuuming, because a
+ * scan which cannot find a truncated heap page cannot determine that the
+ * snapshot is too old to read that page.  We might be able to get away with
+ * truncating all except one of the pages, setting its LSN to (at least) the
+ * maximum of the truncated range if we also treated an index leaf tuple
+ * pointing to a missing heap page as something to trigger the "snapshot too
+ * old" error, but that seems fragile and seems like it deserves its own patch
+ * if we consider it.
+ *
  * This is split out so that we can test whether truncation is going to be
  * called for before we actually do it.  If you change the logic here, be
  * careful to depend only on fields that lazy_scan_heap updates on-the-fly.