Add documentation for issues rate limit

- Add page Rate limits for issues creation
- Add link to it in Rate limits page

doc/security/rate_limits.md

@@ -22,8 +22,9 @@ similarly mitigated by a rate limit.
 
 ## Admin Area settings
 
+- [Issues rate limits](../user/admin_area/settings/rate_limit_on_issues_creation.md).
 - [User and IP rate limits](../user/admin_area/settings/user_and_ip_rate_limits.md).
-- [Rate limits on raw endpoints](../user/admin_area/settings/rate_limits_on_raw_endpoints.md)
+- [Raw endpoints rate limits](../user/admin_area/settings/rate_limits_on_raw_endpoints.md).
 - [Protected paths](../user/admin_area/settings/protected_paths.md).
 
 ## Rack Attack initializer

doc/user/admin_area/settings/rate_limit_on_issues_creation.md

+---
+type: reference
+---
+
+# Rate limits on issue creation
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/55241) in GitLab 12.10.
+
+This setting allows you to rate limit the requests to the issue creation endpoint.
+It defaults to 300 requests per minute.
+You can change it in **Admin Area > Settings > Network > Performance Optimization**.
+
+For example, requests using the
+[Projects::IssuesController#create](https://gitlab.com/gitlab-org/gitlab/raw/master/app/controllers/projects/issues_controller.rb)
+action exceeding a rate of 300 per minute are blocked. Access to the endpoint is allowed after one minute.
+
+![Rate limits on issues creation](img/rate_limit_on_issues_creation.png)
+
+This limit is:
+
+- Applied independently per project and per user.
+- Not applied per IP address.
+- Active by default. To disable it, set the option to `0`.
+
+Requests over the rate limit are logged into the `auth.log` file.