expand on why the futex test skips mach_timebase_info

Author: joboet

tests/pass-dep/concurrency/apple-futex.rs

@@ -87,8 +87,10 @@ fn wait_absolute_timeout() {
     #[allow(deprecated)]
     let mut deadline = unsafe { libc::mach_absolute_time() };
 
-    // Add 200ms (note: this only works under miri because it uses nanosecond units for
-    // mach_absolute_time).
+    // Add 200ms.
+    // What we should be doing here is call `mach_timebase_info` to determine the
+    // unit used for `deadline`, but we know what Miri returns for that function:
+    // the unit is nanoseconds.
     deadline += 200_000_000;
 
     let futex: i32 = 123;