Using @UserRoles function, of course, in hide when, like this:
hide-when for fields (admin always sees them; others see them only if the doc wasn't submitted):
docSubmitted != "" & @IsNotMember( "[ADMIN]"; @UserRoles )
hide-when for computed-text (admin never sees it; others see it only if the doc was submitted):
docSubmitted = "" | @IsMember( "[ADMIN]"; @UserRoles )
where "docSubmitted" is your flag, a hidden, editable, text field, whose value you set to "1" from the Submit action.