[llvm] Improve description of what is considered a security issue (PR #147035)

[Kristof Beyls via llvm-commits]

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@@ -204,10 +205,9 @@ The LLVM Security Policy may be changed by majority vote of the LLVM Security Re
 What is considered a security issue?
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kbeyls wrote:

I quite like the sentence that @wphuhn-intel suggested above as a general principle: "_issues with LLVM that affect developers who consciously chose to use LLVM tooling to generate binaries are considered out-of-scope, issues with LLVM which affect the end users of said developer are in scope._" That being said, I'm not sure if it would correctly cover all the different cases we do document further in detail.

I'd prefer to land this PR without a tl;dr, as is.
We can then see what the feedback is on the this version of the documentation, and add a well-thought-through tl;dr if the feedback is clear that that would be a major improvement.

https://github.com/llvm/llvm-project/pull/147035