Add documentation for incident management rate limits

doc/administration/instance_limits.md

@@ -178,6 +178,18 @@ Plan.default.limits.update!(ci_pipeline_schedules: 100)
 
 ## Instance monitoring and metrics
 
+### Incident Management inbound alert limits
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/14932) in GitLab 12.5.
+
+Limiting inbound alerts for an incident reduces the number of alerts (issues)
+that can be created within a period of time, which can help prevent overloading
+your incident responders with duplicate issues. You can reduce the volume of
+alerts in the following ways:
+
+- Max requests per period per project, 3600 seconds by default.
+- Rate limit period in seconds, 3600 seconds by default.
+
 ### Prometheus Alert JSON payloads
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/14929) in GitLab 12.6.