Rollup merge of rust-lang#63872 - marmistrz:readdir, r=jonas-schievink

Document platform-specific behavior of the iterator returned by std::fs::read_dir

Author: Centril

Diff for: src/libstd/fs.rs

@@ -1956,7 +1956,8 @@ pub fn remove_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// # Platform-specific behavior
 ///
 /// This function currently corresponds to the `opendir` function on Unix
-/// and the `FindFirstFile` function on Windows.
+/// and the `FindFirstFile` function on Windows. Advancing the iterator
+/// currently corresponds to `readdir` on Unix and `FindNextFile` on Windows.
 /// Note that, this [may change in the future][changes].
 ///
 /// [changes]: ../io/index.html#platform-specific-behavior