be even more explicit about what happens when copying os_unfair_lock

Co-authored-by: Ralf Jung <[email protected]>

tests/pass-dep/concurrency/apple-os-unfair-lock.rs

@@ -14,8 +14,10 @@ fn main() {
         libc::os_unfair_lock_assert_not_owner(lock.get());
     }
 
-    // `os_unfair_lock`s can be moved and leaked, even when locked,
-    // as long as no operation is occurring on the lock:
+    // `os_unfair_lock`s can be moved and leaked.
+    // In the real implementation, even moving it while locked is possible
+    // (and "forks" the lock, i.e. old and new location have independent wait queues);
+    // Miri behavior differs here and anyway none of this is documented.
     let lock = lock;
     let locked = unsafe { libc::os_unfair_lock_trylock(lock.get()) };
     assert!(locked);