Document GitLab.com limits for pipeline creation

What does this MR do and why?

Related to #352232

It documents the limit set in gitlab-com/gl-infra/production#7091 (comment 954241223)

This limit is still under a feature flag: #362513 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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