more careful wording for unwinding

Author: RalfJung

src/behavior-considered-undefined.md

@@ -32,7 +32,8 @@ code.
 * Invoking undefined behavior via compiler intrinsics.
 * Executing code compiled with platform features that the current platform
   does not support (see [`target_feature`]).
-* Unwinding into another language.
+* Unwinding from an imported `extern` function into Rust, or from Rust out of an
+  `extern` function, except when explicitly supported by the ABI.
 * Producing an invalid value, even in private fields and locals. "Producing" a
   value happens any time a value is assigned, passed to a function/primitive
   operation or returned from a function/primitive operation.