Add CI/CD max downstream pipelines documentation for GitLab.com
What does this MR do and why?
The application setting at Admin area > Settings > CI/CD > Continuous Integration and Deployment > "Maximum downstream pipelines triggered in a project per user"
will be updated to be 350 for GitLab.com in gitlab-com/gl-infra/production-engineering#25083 (closed). This MR adds the corresponding documentation update.
Related to https://gitlab.com/gitlab-org/gitlab/-/issues/425941.
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Edited by Leaminn Ma