Tweak some comments related to variable-numbered stats in pgstat.c

These comments referred to database objects, but depending on the stats
kind dealt with this may not be true.

Issues found while reviewing a different patch in this area.

Discussion: https://postgr.es/m/ZtXR+CtkEVVE/LHF@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/utils/activity/pgstat.c

@@ -768,7 +768,7 @@ pgstat_report_stat(bool force)
 
 	partial_flush = false;
 
-	/* flush database / relation / function / ... stats */
+	/* flush of variable-numbered stats */
 	partial_flush |= pgstat_flush_pending_entries(nowait);
 
 	/* flush of fixed-numbered stats */
@@ -1342,8 +1342,7 @@ pgstat_delete_pending_entry(PgStat_EntryRef *entry_ref)
 }
 
 /*
- * Flush out pending stats for database objects (databases, relations,
- * functions).
+ * Flush out pending variable-numbered stats.
  */
 static bool
 pgstat_flush_pending_entries(bool nowait)