Please update https://docs.moodle.org/dev/Moodle_4.0_developer_update#Tertiary_navigation with details about the tertiary navigation. Specifically what a plugin developer needs to do to be inline with core activities.

At the moment there is very little detail about the tertiary navigation. As there is no API here we really need to explain the rational behind the changes here, what we're trying to achieve, and what developers should do.