Document placeholder reassignment to blocked users not possible when SSO is enforced
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=540600) </details> <!--IssueSummary end--> 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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