Update schema change event structure in docs

Schema change events have now both set: ddl, and tableChanges. This should be reflected in the docs. I was at first surprised about that (as historically, in case of MySQL, only ddl was populated AFAIR), but I think it's a useful side-effect of the framework migration (to verify, we could compare with the behavior of the legacy implementation by means of enabling it).

We should

• Clarify whether this behavior is intended, and if so, update docs accordingly
• Change docs shown for source.table and db: shown as null, which isn't the case actually
• In case of a table rename, source.table has a comma-separated list of old and new name; instead let's show the old name only, consistent with what we do for Oracle