BUG #14041: transaction_read_only documentation

The following bug has been logged on the website:

Bug reference:      14041
Logged by:          Andras Jako
Email address:      jako.andras@eik.bme.hu
PostgreSQL version: 9.5.1
Operating system:   FreeBSD
Description:

http://www.postgresql.org/docs/9.5/static/hot-standby.html says: "Users will
be able to tell whether their session is read-only by issuing SHOW
transaction_read_only."
http://www.postgresql.org/docs/9.5/static/sql-show.html says: "Available
parameters are documented in Chapter 18 and on the SET reference page."

But I can't find transaction_read_only mentioned nor in Chapter 18 nor on
the SET reference page.