Many of the built-ins are currently turned off, whilst the team continues configuring things and bug-fixing.

The intended built-ins are listed at https://meta.wikimedia.org/wiki/Abstract_Wikipedia/Reserved_...

There are more interesting examples of some user-made functions at the Older Beta-deployment location the team has been using for the past year: https://wikifunctions.beta.wmflabs.org/wiki/Wikifunctions:Ma...