Document CI job activity limit

Provide definition of the limit, an example and
how to set the limit

changelogs/unreleased/docs-job-activity-limit.yml

+---
+title: Document CI job activity limit for pipeline creation
+merge_request: 23246
+author:
+type: added

doc/administration/instance_limits.md

@@ -42,3 +42,35 @@ Activity history for projects and individuals' profiles was limited to one year
 A maximum number of project webhooks applies to each GitLab.com tier. Check the
 [Maximum number of webhooks (per tier)](../user/project/integrations/webhooks.md#maximum-number-of-webhooks-per-tier)
 section in the Webhooks page.
+
+## CI/CD limits
+
+### Number of jobs in active pipelines
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/32823) in GitLab 12.6.
+
+The total number of jobs in active pipelines can be limited per project. This limit is checked
+each time a new pipeline is created. An active pipeline is any pipeline in one of the following states:
+
+- `created`
+- `pending`
+- `running`
+
+If a new pipeline would cause the total number of jobs to exceed the limit, the pipeline
+will fail with a `job_activity_limit_exceeded` error.
+
+- On GitLab.com different [limits are defined per plan](../user/gitlab_com/index.md#gitlab-cicd) and they affect all projects under that plan.
+- On [GitLab Starter](https://about.gitlab.com/pricing/#self-managed) tier or higher self-hosted installations, this limit is defined for the `default` plan that affects all projects.
+  This limit is disabled by default.
+
+To set this limit on a self-hosted installation, run the following in the
+[GitLab Rails console](https://docs.gitlab.com/omnibus/maintenance/#starting-a-rails-console-session):
+
+```ruby
+# If limits don't exist for the default plan, you can create one with:
+# Plan.default.create_limits!
+
+Plan.default.limits.update!(ci_active_jobs: 500)
+```
+
+Set the limit to `0` to disable it.

doc/user/gitlab_com/index.md

@@ -75,6 +75,7 @@ Below are the current settings regarding [GitLab CI/CD](../../ci/README.md).
 | Artifacts maximum size (uncompressed) | 1G                | 100M          |
 | Artifacts [expiry time](../../ci/yaml/README.md#artifactsexpire_in)   | kept forever           | deleted after 30 days unless otherwise specified    |
 | Scheduled Pipeline Cron | `*/5 * * * *` | `*/19 * * * *` |
+| [Max jobs in active pipelines](../../administration/instance_limits.md#number-of-jobs-in-active-pipelines) | `500` for Free tier, unlimited otherwise | Unlimited
 
 ## Repository size limit