MDL-43557 events: fixed minor issues in existing events
Changed notes_viewed event description
Re-added the 'relateduserid' other field when triggering the course_module_completion_updated event
Changed the descriptions for the mod_assign submission_created and submission_updated event
Added description to the \mod_quiz\event\attempt_deleted event
Fixed 'other' docs for events
Fixed book export event URL
Removed any additional validation added in this issue