Update library/core/src/intrinsics.rs

Co-authored-by: Ralf Jung <post@ralfj.de>

library/core/src/intrinsics.rs

@@ -2573,7 +2573,10 @@ extern "rust-intrinsic" {
     /// macro expansion.
     ///
     /// This always returns `false` in const eval and Miri. The interpreter provides better
-    /// diagnostics than the checks that this is used to implement.
+    /// diagnostics than the checks that this is used to implement. However, this means
+    /// you should only be using this intrinsic to guard requirements that, if violated,
+    /// immediately lead to UB. Otherwise, const-eval and Miri will miss out on those
+    /// checks entirely.
     ///
     /// Since this is evaluated after monomorphization, branching on this value can be used to
     /// implement debug assertions that are included in the precompiled standard library, but can