The current Wikibase table documentation seems to be outdated (https://www.mediawiki.org/wiki/Wikibase/Schema ). Maybe more information is available, but that is well hidden.
At least these tables should be documented:
MariaDB [wikidatawiki_p]> show tables LIKE 'wb%';
--------------------------------
| Tables_in_wikidatawiki_p (wb%) |
--------------------------------
| wb_changes |
| wb_changes_dispatch |
| wb_changes_subscription |
| wb_entity_per_page |
| wb_id_counters |
| wb_items_per_site |
| wb_property_info |
| wbc_entity_usage |
| wbs_propertypairs |
--------------------------------
10 rows in set (0.00 sec)
And this documentation should show up in the search when you search for the table name (like "wb_items_per_site")