Notes
Free-form notes on a site.
The site's Notes page holds notes about the site — hosting details, who to call, what not to touch, why a plugin is deactivated.
Administrators write and delete them. Every member who can see the site can read them, clients included, so a note is a good place for anything the customer should also know.
Notes live on the platform, not on the site: they survive a restore of the site and are not part of a backup.