Add example of using a multi-relation in API rules

[csmith]

I was expecting allowed_users ?= @request.auth.id to work based on how the multi-relation is presented in the admin UI, and had to dig up this discussion to figure out how the multi-relation is used: https://github.com/pocketbase/pocketbase/discussions/2157

[ganigeorgiev]

Thanks! The documentation will get a major refactoring with the upcoming v0.17.0 release.