pr review: list all -unwind ABI's in unstable book

 ### Changes

    This is a small commit, updating the `c-unwind` page in the unstable
    book to list _all_ of the other ABI strings that are introduced by
    this feature gate.

    Now, all of the ABI's specified by RFC 2945 are shown.

Co-authored-by: Amanieu d'Antras <[email protected]>

Author: katelyn a. martin

src/doc/unstable-book/src/language-features/c-unwind.md

@@ -6,7 +6,8 @@ The tracking issue for this feature is: [#74990]
 
 ------------------------
 
-Introduces a new ABI string, "C-unwind", to enable unwinding from other
+Introduces four new ABI strings: "C-unwind", "stdcall-unwind",
+"thiscall-unwind", and "system-unwind". These enable unwinding from other
 languages (such as C++) into Rust frames and from Rust into other languages.
 
 See [RFC 2945] for more information.