Enhance act as saas docs

What does this MR do and why?

Related to: !83146 (merged)

This MR clarifies the way how to make a GDK act as SaaS in our docs.

Docs link: https://docs.gitlab.com/ee/development/ee_features.html#act-as-saas

Screenshots or screen recordings

Skipping this because of a trivial change.

How to set up and validate locally

Skipping this because of a trivial change.

MR acceptance checklist

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