Siri does have documentation: https://support.apple.com/en-ca/guide/iphone/ipha48873ed6/io.... This list (recursively) contains more things than probably 95% of users ever do with Siri. The problem really boils down to the fact that a CLI is imposing enough that someone will need a manual (or a teacher), whereas a natural language interface looks like it should support "basically any query" but in practice does not (and cannot) due to fundamental limitations. Those limitations are not obvious, especially to lay users, making it impossible in practice to know what can and cannot be done.