Make default values bold in Synopsis / bnf diagram

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/sql-set-transaction.html
Description:

As an example let's take take "BEGIN"
in the Synopsis :
-------- Start Synopsis for BEGIN --------------------
BEGIN [ WORK | TRANSACTION ] [ transaction_mode [, ...] ]

where transaction_mode is one of:

ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ
UNCOMMITTED }
READ WRITE | READ ONLY
[ NOT ] DEFERRABLE
-------- End Synopsis for BEGIN --------------------
In this example the words to words "READ COMMITTED", "READ WRITE" should be
bold since they are default values if I execute the statement "BEGIN;" see
the example.

By the way I started in "BEGIN" but here the defaults was not even present
sow I went to "START TRANSACTION but still no indication on default values
but finally in "SET TRANSACTION"
There are default value for "ISOLATION LEVEL" but nothing on default value
for "READ WRITE | READ ONLY" or "[ NOT ] DEFERRABLE".