Add PHP doc for events

[MorrisJobke]

Streamlines them with the documentation page so that we can extract them automatically at some time.

See nextcloud/documentation#2842

[faily-bot]

🤖 beep boop beep 🤖

Here are the logs for the failed build:

Status of 31558: failure

jsunit

Show full log

PhantomJS 2.1.1 (Linux 0.0.0) LOG: 'JQMIGRATE: Migrate is installed, version 1.4.1'
PhantomJS 2.1.1 (Linux 0.0.0) WARN: 'jQuery is deprecated: The global jQuery is deprecated. It will be updated to v3.x in Nextcloud 21. In later versions of Nextcloud it might be removed completely. Please ship your own.'
PhantomJS 2.1.1 (Linux 0.0.0) WARN: '$ is deprecated: The global jQuery is deprecated. It will be updated to v3.x in Nextcloud 21. In later versions of Nextcloud it might be removed completely. Please ship your own.'
PhantomJS 2.1.1 (Linux 0.0.0) WARN: 'Handlebars is deprecated: please ship your own, this will be removed in Nextcloud 20'
PhantomJS 2.1.1 (Linux 0.0.0) DEBUG: 'OCA.Files.Settings initialized'
PhantomJS 2.1.1 (Linux 0.0.0) DEBUG: 'OCA.Files.Sidebar initialized'
PhantomJS 2.1.1 (Linux 0.0.0) DEBUG: 'OCA.Sharing.ShareSearch initialized'
PhantomJS 2.1.1 (Linux 0.0.0) DEBUG: 'OCA.Sharing.ExternalLinkActions initialized'
PhantomJS 2.1.1 (Linux 0.0.0): Executed 847 of 850 (skipped 3) SUCCESS (13.628 secs / 12.998 secs)