In order to add an option to your help string, you must provide a list beginning with the option type and followed by key/value pairs describing the option’s properties. For instance, to add a flag with a short name and a description, you could do this:
(flag :short-name "h" :description "Print this help and exit.")
Option properties vary depending on the option type. The exact list of available option types, and the corresponding properties are described in Built-In Option Types.