Document subdomain to domain change for 4.0 (#2135)

* Document subdomain to domain change for 4.0

Documents prebid/Prebid.js#5500

* Update userId.md

* tweaked wording

Co-authored-by: bretg <bgorsline@gmail.com>

dev-docs/modules/userId.md

@@ -45,6 +45,8 @@ Note that User IDs aren't needed in the mobile app world because device ID is av
 
 Also note that not all bidder adapters support all forms of user ID. See the tables below for a list of which bidders support which ID schemes.
 
+As of Prebid 4.0, this module will attempt storage in the main domain of the publisher's website instead of a subdomain, unless this behavior is overriden by a submodule.
+
 ## User ID, GDPR, and Opt-Out
 
 When paired with the [Consent Management](/dev-docs/modules/consentManagement.html) module, privacy rules are enforced: