add comment about emulation deficiencies

Author: joboet

src/shims/unix/macos/sync.rs

@@ -3,7 +3,12 @@
 //! For `os_unfair_lock`, see the documentation
 //! <https://developer.apple.com/documentation/os/synchronization?language=objc>
 //! and in case of underspecification its implementation
-//! <https://github.com/apple-oss-distributions/libplatform/blob/a00a4cc36da2110578bcf3b8eeeeb93dcc7f4e11/src/os/lock.c#L645>
+//! <https://github.com/apple-oss-distributions/libplatform/blob/a00a4cc36da2110578bcf3b8eeeeb93dcc7f4e11/src/os/lock.c#L645>.
+//!
+//! Note that we don't emulate every edge-case behaviour of the locks. Notably,
+//! we don't abort when locking a lock owned by a thread that has already exited
+//! and we do not detect copying of the lock, but macOS doesn't guarantee anything
+//! in that case either.
 
 use crate::*;