Add PG18 compatibility note for psql "\d" query optimization in regression tests

src/test/regress/expected/multi_mx_hide_shard_names.out

@@ -109,6 +109,7 @@ EXPLAIN (COSTS OFF) INSERT INTO pg_class SELECT * FROM pg_class;
 
 -- Check that query that psql "\d test_table" does gets optimized to an index
 -- scan
+-- PG18+: the psql \d-style regex on pg_class can flip the plan to a Seq Scan
 EXPLAIN (COSTS OFF) SELECT c.oid,
   n.nspname,
   c.relname

src/test/regress/expected/multi_mx_hide_shard_names_0.out

@@ -109,6 +109,7 @@ EXPLAIN (COSTS OFF) INSERT INTO pg_class SELECT * FROM pg_class;
 
 -- Check that query that psql "\d test_table" does gets optimized to an index
 -- scan
+-- PG18+: the psql \d-style regex on pg_class can flip the plan to a Seq Scan
 EXPLAIN (COSTS OFF) SELECT c.oid,
   n.nspname,
   c.relname

src/test/regress/sql/multi_mx_hide_shard_names.sql

@@ -60,6 +60,7 @@ EXPLAIN (COSTS OFF) INSERT INTO pg_class SELECT * FROM pg_class;
 
 -- Check that query that psql "\d test_table" does gets optimized to an index
 -- scan
+-- PG18+: the psql \d-style regex on pg_class can flip the plan to a Seq Scan
 EXPLAIN (COSTS OFF) SELECT c.oid,
   n.nspname,
   c.relname