Add reference to Rust issue 46188

There's a long history of discussion on how incorrect declarations in
`extern` blocks might cause UB in programs compiled using LLVM.  Let's
link to one of the issues in that history.

(Thanks to madsmtm for raising this question, and to RalfJ for
providing this citation.)

Author: traviscross

text/0000-unsafe-extern-blocks.md

@@ -11,11 +11,13 @@ In Edition 2024 it is `unsafe` to declare an `extern` function or static, but ex
 # Motivation
 [motivation]: #motivation
 
-Simply declaring extern items, even without ever using them, can cause Undefined Behavior.
+Simply declaring extern items, even without ever using them, can cause Undefined Behavior (see, e.g., issue [#46188][]).
 When performing cross-language compilation, attributes on one function declaration can flow to the foreign declaration elsewhere within LLVM and cause a miscompilation.
 In Rust we consider all sources of Undefined Behavior to be `unsafe`, and so we must make declaring extern blocks be `unsafe`.
 The up-side to this change is that in the new style it will be possible to declare an extern fn that's safe to call after the initial unsafe declaration.
 
+[#46188]: https://github.com/rust-lang/rust/issues/46188
+
 # Guide-level explanation
 [guide-level-explanation]: #guide-level-explanation