transction_timestamp() inside of procedures

This function shows that only clock_timestamp() advances inside a
procedure, not statement_timestamp() or transaction_timestamp():

    CREATE OR REPLACE PROCEDURE test_timestamp () AS $$
    DECLARE
            str TEXT;
    BEGIN
            WHILE TRUE LOOP
                    -- clock_timestamp() is updated on every loop
                    SELECT clock_timestamp() INTO str;
                    RAISE NOTICE 'clock       %', str;
                    SELECT statement_timestamp() INTO str;
                    RAISE NOTICE 'statement   %', str;
                    SELECT transaction_timestamp() INTO str;
                    RAISE NOTICE 'transaction %', str;
                    COMMIT;
    
                    PERFORM pg_sleep(2);
            END LOOP;
    END
    $$ LANGUAGE plpgsql;

    CALL test_timestamp();
    NOTICE:  clock       2018-09-20 19:38:22.575794-04
    NOTICE:  statement   2018-09-20 19:38:22.575685-04
    NOTICE:  transaction 2018-09-20 19:38:22.575685-04

-->    NOTICE:  clock       2018-09-20 19:38:24.578027-04
    NOTICE:  statement   2018-09-20 19:38:22.575685-04
    NOTICE:  transaction 2018-09-20 19:38:22.575685-04

This surprised me since I expected a new timestamp after commit.  Is
this something we want to change or document?  Are there other
per-transaction behaviors we should adjust?

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +