A table of magic constants

Hello,

My reason/excuse for asking what I'll eventually ask is that I'm a new
PostgreSQL practitioner so the amount of things I don't know about
PostgreSQL is depressingly large.

While reading through the recent "Row level security - notes and questions"
thread I saw this SQL statement:

CREATE POLICY accounts_policy ON testrls.accounts
FOR ALL
TO users
USING (true)
WITH CHECK (username = SESSION_USER);

The bit that is the catalyst for this e-mail is: SESSION_USER.

As a recent convert to the Church of Postgres I've been consuming vast
amounts of information on PostgreSQL, and SESSION_USER is not the first nor
only, what I'm calling magic constant, that I've seen. Off the top of my
head, other examples that I've encountered are CURRENT_USER and
CURRENT_TIMESTAMP.

So my question is this, is there a reference table in the documentation
that I haven't found yet that lists all magic constants and their meaning?
And if not in the official documentation is it in the wiki?

Thanks,

Dane