Adds a reference for CodeQL Ast module classes and associated Ruby syntax.

The Ruby syntax provided is not complete. I used parse.y as a reference, and the actual syntax is very complex and not really realistic to present in a table like this. What's here should hopefully be enough to give a flavour of the kinds of expressions that map onto the provided CodeQL classes.

In some cases where it seemed clearer to do so, such as the Literals section, I've resorted to providing example expressions rather than a syntax definition. It may be that this approach would be better for some other sections as well.