Fix hash index vs "snapshot too old" problemms

Hash indexes are not WAL-logged, and so do not maintain the LSN of
index pages.  Since the "snapshot too old" feature counts on
detecting error conditions using the LSN of a table and all indexes
on it, this makes it impossible to safely do early vacuuming on any
table with a hash index, so add this to the tests for whether the
xid used to vacuum a table can be adjusted based on
old_snapshot_threshold.

While at it, add a paragraph to the docs for old_snapshot_threshold
which specifically mentions this and other aspects of the feature
which may otherwise surprise users.

Problem reported and patch reviewed by Amit Kapila

src/backend/utils/time/snapmgr.c

@@ -1590,7 +1590,8 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
 		&& old_snapshot_threshold >= 0
 		&& RelationNeedsWAL(relation)
 		&& !IsCatalogRelation(relation)
-		&& !RelationIsAccessibleInLogicalDecoding(relation))
+		&& !RelationIsAccessibleInLogicalDecoding(relation)
+		&& !RelationHasUnloggedIndex(relation))
 	{
 		int64		ts = GetSnapshotCurrentTimestamp();
 		TransactionId xlimit = recentXmin;