Add the feature flag changes to the trigger list for as-if-jh

Context

While reading docs to prepare for working on #386585 (closed), I noticed that we have two sources for when the JiHu validation pipeline will be triggered:

• https://about.gitlab.com/handbook/ceo/chief-of-staff-team/jihu-support/jihu-validation-pipelines.html#when-and-where-we-run-it
• https://docs.gitlab.com/ee/development/pipelines/index.html#as-if-jh-cross-project-downstream-pipeline

Those two sources do not say the same thing.

After checking the rules we have in place, I think we need to update the docs.

What does this MR do and why?

Update the list of triggers for pipeline:run-as-if-jh in the docs.

A note on SSOT

We should ideally have one source of truth for that list, and crosslink it in the other reference. I'm not sure which file should be the SSOT. @godfat-gitlab WDYT?

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