Improve documentation on the fake command

I found myself wanting a city, which if you search on tera will possibly turn up `city()` but in our case it depends on fake which then has functions registered. It was really hard to figure this out.

Should figure out where to document what is in the [functions registered in this repo](https://github.com/couchbaselabs/couchbase-shell/blob/772204087d7d2b2f9d9b6534550903a754c12267/src/cli/fake_data/mod.rs#L417-L474), or point to authoritative documentation if we're taking it all.