Update user REST APIs docs to better indicate SaaS availability

What does this MR do and why?

Generated from this thread on the forum.

I noticed that many of the user related REST APIs are noted to be available only to administrators, hence not available on GitLab SaaS, and were missing the "badge" on the title to better inticate that.

With this MR I'm trying to improve the docs to make that clearer 🙂

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