Add docs linking to the 1MB limit for alerts

doc/administration/instance_limits.md

@@ -219,6 +219,12 @@ alerts in the following ways:
 
 Prometheus alert payloads sent to the `notify.json` endpoint are limited to 1 MB in size.
 
+### Generic Alert Alert JSON payloads
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/16441) in GitLab 12.4.
+
+Alert payloads sent to the `notify.json` endpoint are limited to 1 MB in size.
+
 ## Environment data on Deploy Boards
 
 [Deploy Boards](../user/project/deploy_boards.md) load information from Kubernetes about

doc/user/project/integrations/generic_alerts.md

@@ -40,6 +40,9 @@ You can customize the payload by sending the following parameters. All fields ar
 | `hosts` | String or Array | One or more hosts, as to where this incident occurred. |
 | `severity` | String | The severity of the alert. Must be one of `critical`, `high`, `medium`, `low`, `info`, `unknown`. Default is `critical`. |
 
+TIP: **Payload size:**
+Ensure you are within the [payload application limits](../../../administration/instance_limits.md#generic-alert-alert-json-payloads) when making your request.
+
 Example request:
 
 ```shell