restore invalid unwinding in Rust bullet; make more explicit

Author: BatmanAoD

src/behavior-considered-undefined.md

@@ -58,7 +58,9 @@ Please read the [Rustonomicon] before writing unsafe code.
 * Invoking undefined behavior via compiler intrinsics.
 * Executing code compiled with platform features that the current platform
   does not support (see [`target_feature`]), *except* if the platform explicitly documents this to be safe.
-* Calling a function with the wrong [call ABI][abi].
+* Calling a function with the wrong [call ABI][abi], or unwinding past a stack
+  frame that does not allow unwinding (e.g. by calling a `"C-unwind"` function
+  imported or transmuted as a `"C"` function or function pointer).
 * Producing an [invalid value][invalid-values]. "Producing" a
   value happens any time a value is assigned to or read from a place, passed to
   a function/primitive operation or returned from a function/primitive