sys/unix: add comments for some Miri fallbacks

Author: RalfJung

library/std/src/sys/pal/unix/fs.rs

@@ -899,7 +899,7 @@ impl DirEntry {
             target_os = "android",
             target_os = "hurd"
         ),
-        not(miri)
+        not(miri) // no dirfd on Miri
     ))]
     pub fn metadata(&self) -> io::Result<FileAttr> {
         let fd = cvt(unsafe { dirfd(self.dir.dirp.0) })?;

library/std/src/sys/sync/thread_parking/mod.rs

@@ -23,6 +23,7 @@ cfg_if::cfg_if! {
         mod windows7;
         pub use windows7::Parker;
     } else if #[cfg(all(target_vendor = "apple", not(miri)))] {
+        // Doesn't work in Miri, see <https://github.com/rust-lang/miri/issues/2589>.
         mod darwin;
         pub use darwin::Parker;
     } else if #[cfg(target_os = "xous")] {