Update comment

src/test/regress/expected/multi_modifications.out

@@ -327,7 +327,7 @@ SELECT symbol, bidder_id FROM limit_orders WHERE id = 246;
 UPDATE limit_orders SET bidder_id = id WHERE id = 246;
 -- updates referencing a column are supported
 UPDATE limit_orders SET bidder_id = bidder_id + 1 WHERE id = 246;
--- pure functions are allowed
+-- IMMUTABLE functions are allowed
 UPDATE limit_orders SET symbol = LOWER(symbol) WHERE id = 246;
 SELECT symbol, bidder_id FROM limit_orders WHERE id = 246;
  symbol | bidder_id 
@@ -335,7 +335,7 @@ SELECT symbol, bidder_id FROM limit_orders WHERE id = 246;
  gm     |       247
 (1 row)
 
--- updates referencing an unpure function are unsupported
+-- updates referencing non-IMMUTABLE function are unsupported
 UPDATE limit_orders SET placed_at = now() WHERE id = 246;
 ERROR:  cannot plan sharded modification containing values which are not constants or constant expressions
 -- cursors are not supported

src/test/regress/sql/multi_modifications.sql

@@ -243,12 +243,12 @@ UPDATE limit_orders SET bidder_id = id WHERE id = 246;
 -- updates referencing a column are supported
 UPDATE limit_orders SET bidder_id = bidder_id + 1 WHERE id = 246;
 
--- pure functions are allowed
+-- IMMUTABLE functions are allowed
 UPDATE limit_orders SET symbol = LOWER(symbol) WHERE id = 246;
 
 SELECT symbol, bidder_id FROM limit_orders WHERE id = 246;
 
--- updates referencing an unpure function are unsupported
+-- updates referencing non-IMMUTABLE functions are unsupported
 UPDATE limit_orders SET placed_at = now() WHERE id = 246;
 
 -- cursors are not supported