You seem to have found an occult Looker secret, or at least a reference to one. I’d start looking over your shoulder, if I were you… The truth is out there.
But really, it does look like we have not documented that— The feature was borne from this discussion, which has some explanation:
AFAIK, in earlier days of BigQuery, join each was a somewhat sad fact of life due to performance restrictions, but now is generally pretty unnecessary, so perhaps we don’t want to encourage its use by including it in the docs.
I’ll connect with our docs team on whether or not we’ll document this or remove the reference to it entirely.