While removing all locations on Content makes sense, it does not make much sense on users as user groups are tied to permissions.

So this enhancement is about:

• Adding API doc for UserService->unAssignUserFromUserGroup() to throw BadState when trying to remove last user group
• Change Core impl to enforce this, and update phpdoc of the other implementations
• Add doc/bc/5.1/changes.md like in ezpublish-legacy to add a note on the bc break