reference: document unwinding unsafety issues
@@ -1171,6 +1171,9 @@ exposing an API making it possible for it to occur in safe code.
* A discriminant in an `enum` not included in the type definition
* A value in a `char` which is a surrogate or above `char::MAX`
* non-UTF-8 byte sequences in a `str`
+* Unwinding into Rust from foreign code or unwinding from Rust into foreign
+ code. Rust's failure system is not compatible with exception handling in
+ other languages. Unwinding must be caught and handled at FFI boundaries.
##### Behaviour not considered unsafe
