Ok here is the solution I found that may help others :
1- install the TAGS storage location plugin (https://www.seblod.com/store/extensions/48650)
2-installe the SQL pack ( https://www.seblod.com/store/extensions/19162)
3-in your "associated docs" list use the SQL live plugin with the following SQL query (button 2)
SELECT GROUP_CONCAT(tag_id) FROM #__contentitem_tag_map WHERE content_item_id=$uri->get('id')
4- in the same liste (button 3) use "any word exact' with the "qupted" comparison rule and simple comparison mode
now your list displays the documents that share at least one tag with the current article !!
Cyril