[clang] [BoundsSafety][doc] Make it clear that the feature is work-in-progress (PR #95964)

[Aaron Ballman via cfe-commits]

================
@@ -8,6 +8,9 @@
 Overview
 ========
 
+**NOTE:** This is a design document and the feature is not available for users yet.
+Please find :doc:`BoundsSafetyImplPlans` for more details.
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AaronBallman wrote:

```suggestion
Please see :doc:`BoundsSafetyImplPlans` for more details.
```

https://github.com/llvm/llvm-project/pull/95964