I have updated the diagram from the source xml files that ship with Moodle 3.3 .You can see the diagram here
And you can read about the purpose and potential benefit here
http://www.examulator.com/er/er_faq.html
I would appreciate suggestions for pk/fk relationships that are not in the source but reflect the relationships in the data. For example it looks likely that there should be a one to many relationship between tables wiki_pages and wiki_synonyms and wiki_subwikis and wiki_synonyms.
This relationship (or lack of) can be seen in this diagram.