This article (kb 000004048) should clarify how. Note that you'd need a Publisher license for this functionality to be enabled and that only individual users can be used, not groups.
Document Administrators are able to manage User Documents and Source Documents (depending on where they are set up). So they can manage Document CALs as you were looking for.
What they cannot do, is manage licenses in general (server license), or Named User CALs.
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