Clarify the functionality of protected CI variables

What does this MR do?

We have a long-standing issue of people misunderstanding what it means to "protect" a variable. I rewrote some of our documentation to focus on the link to protected refs, to try to reduce the impression that "protection" offers some extra layer of encryption, or keeps the value of the variable secret from people who use it.

We've had multiple discussions in Verify about how this kind of security is impossible to provide.

I'm hoping the changes I've made clarify that point, because I'm closing that issue either way and I'd like to make this concept easy to understand.

Related issues

#28002 (closed)

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Resolves #28002 (closed)