Document reviews are currently stored in the Management Review module, but I find it slightly confusing. Also, the Management Review module fills up now with a variety of records - many of which are not as relevant as Management Reviews - which makes the access your relevant information trickier.
There is clearly a well-thought logic to storing all different reviews in the same module, but at the same time it should not be followed if an alternative logic would create a more intuitive work environment. Keeping Documents and Document Reviews in the same module would create a central place where you know you can do ALL your document related tasks and where you can find ALL your document related information during an audit.
I think a great way improve the situation would be to implement three changes.
My four suggestions
1) Add a button in the Document module for "Add Document Review"
2) Add a "Complete" tab in the Document module that shows completed document reviews
3) Bring open document review actions into the "Actions" tab in Document module
4) Extend the reports with necessary reports