std: mention remove_dir_all can emit DirectoryNotEmpty when concurrently written into

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

library/std/src/fs.rs

@@ -2874,6 +2874,8 @@ pub fn remove_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
 ///
 /// Consider ignoring the error if validating the removal is not required for your use case.
 ///
+/// This function may return [`io::ErrorKind::DirectoryNotEmpty`] if the directory is concurrently
+/// written into, which typically indicates some contents were removed but not all.
 /// [`io::ErrorKind::NotFound`] is only returned if no removal occurs.
 ///
 /// [`fs::remove_file`]: remove_file