Here: http://docs.textadventures.co.uk/quest/commands.html

Here's the section:

To add a command:

In the Windows desktop version, select “game” in the tree. Now, you can right-click on the tree and choose “Add Command”, or use the Add menu and choose “Command”.

In the web version, select “Commands” in the tree (underneath “game”). Then click the “Add” button.

The pattern we will match against is “help”, and we just want to print something, so it will just look like this:

[[[THIS IS THE MISSING CHUNK!]]]

Now if you play the game and type HELP, you will see the new text.

What's missing here?

Not so much a chunk of documentation; all that's missing is a screenshot.

I checked in the repo, and it looks like that image is missing entirely (not just a misspelled filename or something); but I'm guessing it will just show what the text above it says, so shouldn't be hard to create a new one.