Rollup merge of rust-lang#138773 - RalfJung:catch_unwind_docs, r=jhpratt

catch_unwind intrinsic: document return value

Seems like we forgot to document this. The comment reflects what Miri does, which seems to also match what codegen does at least [in `codegen_gnu_try`](https://github.com/rust-lang/rust/blob/b754ef727ca87050a8d758fc44f524cfb4310eff/compiler/rustc_codegen_llvm/src/intrinsic.rs#L953-L964).

Author: matthiaskrgr

core/src/intrinsics/mod.rs

@@ -3002,6 +3002,7 @@ pub const fn discriminant_value<T>(v: &T) -> <T as DiscriminantKind>::Discrimina
 
 /// Rust's "try catch" construct for unwinding. Invokes the function pointer `try_fn` with the
 /// data pointer `data`, and calls `catch_fn` if unwinding occurs while `try_fn` runs.
+/// Returns `1` if unwinding occurred and `catch_fn` was called; returns `0` otherwise.
 ///
 /// `catch_fn` must not unwind.
 ///