Mention -unwind ABI variants #3729

As of Rust 1.71, every ABI except for Rust comes in two variants, an -unwind version and the original non-unwind version. This isn't something the book should cover in detail, but it seems reasonable to mention briefly.

The most appropriate place is probably Chapter 19, towards the end of "Using extern Functions to Call External Code". I'm not entirely sure whether it deserves a full paragraph or even its own inset, but here's some draft verbiage:

There are also ABI strings with -unwind, such as "C-unwind". The "unwind" in these strings refers to panic in Rust and to exceptions in C++. It is usually correct to use the standard, non-unwind version of an ABI string, which will cause the program to immediately exit with an error rather than letting a Rust panic or C++ exception "escape" from one language runtime to the other. Unwinding across language boundaries is hazardous and generally not a good way to expose errors in a cross-language API. If you do need to unwind across an FFI boundary, refer to the Reference or the Nomicon to learn how to do so safely.

If that's too much detail/information, it could be further shortened to:

There are also ABI strings with -unwind, such as "C-unwind". It is usually correct to use the standard, non-unwind version of an ABI string, but if you do need to unwind across an FFI boundary, refer to the Reference or the Nomicon to learn how to do so safely.