Update handlers documentation

**Feature request**
It would be great to have logging for handlers.

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

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

No files or tests are named. Start by locating the handler implementation, configuration parsing, and existing logging entry points; determine what handler events and output are expected. Done means an `enable_logging` option defaults to `false` and, when enabled, provides useful handler logging for production deployments.

Written by the indexing model from the issue text.