fix rustc_nonnull_optimization_guaranteed docs

Author: RalfJung

compiler/rustc_const_eval/src/interpret/call.rs

@@ -234,6 +234,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         // so we implement a type-based check that reflects the guaranteed rules for ABI compatibility.
         if self.layout_compat(caller_abi.layout, callee_abi.layout)? {
             // Ensure that our checks imply actual ABI compatibility for this concrete call.
+            // (This can fail e.g. if `#[rustc_nonnull_optimization_guaranteed]` is used incorrectly.)
             assert!(caller_abi.eq_abi(callee_abi));
             Ok(true)
         } else {

compiler/rustc_feature/src/builtin_attrs.rs

@@ -863,8 +863,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(
         rustc_nonnull_optimization_guaranteed, Normal, template!(Word), WarnFollowing,
         EncodeCrossCrate::Yes,
-        "the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to enable \
-        niche optimizations in libcore and libstd and will never be stable",
+        "the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to document \
+        guaranteed niche optimizations in libcore and libstd and will never be stable
+        (note that the compiler does not even check whether the type indeed is being non-null-optimized;
+        it is your responsibility to ensure that the attribute is only used on types that are optimized)",
     ),
 
     // ==========================================================================