Clarify that unnamed constants are always evaluated at compile time

See rust-lang/rust#93838
It seems like everyone is onboard with blessing this behavior as stable.

Author: XrXr

src/items/constant-items.md

@@ -89,6 +89,18 @@ m!(const _: () = (););
 // const _: () = ();
 ```
 
+Unnamed constants are always [evaluated][const_eval] at compile-time to surface
+panics. They are evaluated at compile-time even when placed within
+an unused function:
+
+```rust,compile_fail
+fn unused_generic_function<T>() {
+    // A failing compile-time assertion
+    const _: () = assert!(usize::BITS == 0);
+}
+```
+
+[const_eval]: ../const_eval.md
 [associated constant]: ../items/associated-items.md#associated-constants
 [constant value]: ../const_eval.md#constant-expressions
 [free]: ../glossary.md#free-item