In the old document module it was possible to mark a document,make a comment, add @menbers and in one click publish it on the Project overview.
I can not find this in the new.
It is not enougth th make a comment on the document.
At the moment you can use "copy link" from under the "share" menu in the detail pane for a document. This will give you the direct link for the document to copy and paste to the Project Overview.
You can also choose the document from the "attach file" option when you post something on the Project Overview.