[docs] Update Navigation API docs with Secondary navigation info

[kabalin]

What is the Moodle feature that needs documenting?

Navigation API needs to be reviewed and updated to use the terminology of Primary and Secondary navigation and contain information how to work with secondary navigation. Some content from from 4.0 Developer Update document can be reused for this.

Is this documentation specific to a Moodle version?

Yes

Are you able to provide a patch for this?

None