Document subdomain to domain change for 4.0 (#2135)
Document subdomain to domain change for 4.0 (#2135)

* Document subdomain to domain change for 4.0

Documents https://github.com/prebid/Prebid.js/pull/5500

* Update userId.md

* tweaked wording

Co-authored-by: bretg <9df09da23577b88db5b6b8911d0635d4f870e78e@gmail.com>
prebid/prebid.github.io,prebid/prebid.github.io,prebid/prebid.github.io