Update validation documentation

**Feature request**
It would be great to have notifications for middleware.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_notifications` with default `false`.

Start by locating the middleware handling and configuration entry points in the repository, then determine how production notifications are currently represented, if at all. Done means a documented `enable_notifications` option defaults to `false` and the requested middleware notifications work as described.

Written by the indexing model from the issue text.