Add a criteria about pipeline age in the review docs

What does this MR do and why?

Related to #386752 (closed)

To prevent some master-broken incident (e.g. gitlab-org/quality/engineering-productivity/master-broken-incidents#379 (comment 1222574915)), we will make sure that people merging MRs with pipelines that have been created more than 6 hours ago get a mention in their MRs, thanks to gitlab-org/quality/triage-ops!1856 (merged).

This MR is here to change the review docs as well to be aligned with that change.

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