doc: Describe Rules and Version for Process Event

The "Process Event" section was a bit outdated, still referring the
removed Icinga 2 source. Especially the added rules and rule version
logic needed some explanation.

Author: oxzi

doc/20-HTTP-API.md

@@ -5,14 +5,24 @@ via `listen` and `debug-password`.
 
 ## Process Event
 
-One possible source next to the Icinga 2 API is event submission to the Icinga Notifications HTTP API listener.
-After creating a source with type _Other_ in Icinga Notifications Web,
+Events can be submitted to Icinga Notifications using the `/process-event` HTTP API endpoint.
+
+After creating a source in Icinga Notifications Web,
 the specified credentials can be used for HTTP Basic Authentication of a JSON-encoded
-[Event](https://github.com/Icinga/icinga-notifications/blob/main/internal/event/event.go).
+[`Event`](https://github.com/Icinga/icinga-go-library/blob/main/notifications/event/event.go).
 
 The authentication is performed via HTTP Basic Authentication, expecting `source-${id}` as the username,
 `${id}` being the source's `id` within the database, and the configured password.
 
+The event is sent to match specific Icinga Notifications rules, created in the web interface.
+Next to an array of `rule_ids`, a `rules_version` must be provided, ensuring that the source has no outdated state.
+
+When the submitted `rules_version` is either outdated or empty, the `/process-event` endpoint returns an HTTP 412 response.
+The response's body is a JSON-encoded version of the
+[`RulesInfo`](https://github.com/Icinga/icinga-go-library/blob/main/notifications/source/client.go),
+containing the latest `rules_version` together with all rules for this source.
+After reevaluating these rules, one can resubmit the event with the updated `rules_version`.
+
 ```
 curl -v -u 'source-2:insecureinsecure' -d '@-' 'http://localhost:5680/process-event' <<EOF
 {
@@ -36,7 +46,7 @@ curl -v -u 'source-2:insecureinsecure' -d '@-' 'http://localhost:5680/process-ev
   "severity": "crit",
   "username": "",
   "message": "Something went somewhere very wrong.",
-  "rule_version": "23",
+  "rules_version": "23",
   "rule_ids": ["0"]
 }
 EOF