I was looking for the spec expected by redis-cli -u since https://github.com/informatikr/hedis doesn't interpret it the same way as redis-cli (hedis will parse password as the username in redis://password@localhost while redis-cli uses it as password-) but I could not find it anywhere.

I found this resource but I am not sure whether it's up to date/ https://www.iana.org/assignments/uri-schemes/prov/redis and the best one imho:
https://github.com/lettuce-io/lettuce-core/wiki/Redis-URI-and-connection-details#uri-syntax

I wish something as fundamental were more clearly documented and cherry on the cake if you can reference doc sections with anchors: