content/docs/alerting: Add reference to Alertmanager API v2

Alertmanager v0.16.0 introduces a new API, v2, replacing API v1 in the
future. This patch adds a reference to the new API in the alerting docs.
Signed-off-by: Max Leonard Inden <IndenML@gmail.com>

content/docs/alerting/clients.md

@@ -7,17 +7,32 @@ nav_icon: sliders
 # Sending alerts
 
 __**Disclaimer**: Prometheus automatically takes care of sending alerts
-generated by its configured [alerting rules](../../prometheus/latest/configuration/alerting_rules/). It is highly
-recommended to configure alerting rules in Prometheus based on time series
-data rather than implementing a direct client.__
+generated by its configured [alerting
+rules](../../prometheus/latest/configuration/alerting_rules/). It is highly
+recommended to configure alerting rules in Prometheus based on time series data
+rather than implementing a direct client.__
 
-The Alertmanager listens for alerts on an API endpoint at `/api/v1/alerts`.
+The Alertmanager has two APIs, v1 and v2, both listening for alerts. The scheme
+for v1 is described in the code snipped below. The scheme for v2 is specified as
+an OpenAPI specification that can be found in the [Alertmanager
+repository](https://github.com/prometheus/alertmanager/blob/master/api/v2/openapi.yaml).
 Clients are expected to continuously re-send alerts as long as they are still
-active (usually on the order of 30 seconds to 3 minutes).
-Clients can push a list of alerts to that endpoint via a POST request of
-the following format:
+active (usually on the order of 30 seconds to 3 minutes). Clients can push a
+list of alerts to Alertmanager via a POST request.
 
-```
+The labels of each alert are used to identify identical instances of an alert
+and to perform deduplication. The annotations are always set to those received
+most recently and are not identifying an alert.
+
+Both `startsAt` and `endsAt` timestamp are optional. If `startsAt` is omitted,
+the current time is assigned by the Alertmanager. `endsAt` is only set if the
+end time of an alert is known. Otherwise it will be set to a configurable
+timeout period from the time since the alert was last received.
+
+The `generatorURL` field is a unique back-link which identifies the causing
+entity of this alert in the client.
+
+```json
 [
   {
     "labels": {
@@ -34,15 +49,3 @@ the following format:
   ...
 ]
 ```
-
-The labels are used to identify identical instances of an alert and to perform
-deduplication. The annotations are always set to those received most recently
-and are not identifying an alert.
-
-Both timestamps are optional. If `startsAt` is omitted, the current time
-is assigned by the Alertmanager. `endsAt` is only set if the end time of an
-alert is known. Otherwise it will be set to a configurable timeout period from
-the time since the alert was last received.
-
-The `generatorURL` field is a unique back-link which identifies the causing
-entity of this alert in the client.