Document placeholder reassignment to blocked users not possible when SSO is enforced
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If an owner of an enterprise group wants to reassign to inactive users after migrating to `.com` but group and group SSO is enforced, inactive users would [fail the SSO enforcement check](https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/ee/import/source_users/reassign_service.rb#L17) if they have [not yet signed in and linked their SAML identity](https://docs.gitlab.com/user/project/import/#requirements). This exception should be documented for enterprise group owner bypass.
Originally from a tread on https://gitlab.com/gitlab-org/gitlab/-/merge_requests/189693: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/189693#note_2488378297
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