document: reload prometheus when deploying alerting rules

See prometheus-alerts!21.

howto/prometheus.md

@@ -115,7 +115,8 @@ The last push command should show you the URL where you can submit
 your merge request.
 
 After being merged, the changes should propagate within [4 to 6
-hours](https://gitlab.torproject.org/tpo/tpa/team/-/wikis/howto/puppet/#cron-and-scheduling).
+hours](https://gitlab.torproject.org/tpo/tpa/team/-/wikis/howto/puppet/#cron-and-scheduling). Prometheus automatically reloads those rules when they are
+deployed.
 
 See also the [targets.d documentation in the git repository](https://gitlab.torproject.org/tpo/tpa/prometheus-alerts/-/tree/main/targets.d).
 
@@ -236,10 +237,15 @@ targets regularly. Alerts can be added through that repository by
 adding a file in the `rules.d` directory, see [rules.d](https://gitlab.torproject.org/tpo/tpa/prometheus-alerts/-/tree/main/rules.d) directory
 for more documentation on that.
 
-Note that alerts (probably?) do not take effect until a sysadmin
-reloads Prometheus.
-
-TODO: confirm how rules are deployed.
+After being merged, the changes should propagate within [4 to 6
+hours](https://gitlab.torproject.org/tpo/tpa/team/-/wikis/howto/puppet/#cron-and-scheduling). Prometheus does *not* automatically reload those rules when
+they are deployed, but Puppet should reload the service after
+deploying the rules.
+
+Note: that wasn't tested. It's possible this doesn't work, see the
+`vcsrepo` resource in `tor-puppet.git`,
+`modules/profile/manifests/prometheus/server/server.pp` for the
+`notify` directive.
 
 ### Adding alert recipients