std: mention remove_dir_all can emit DirectoryNotEmpty when concurrently written into

[xizheyin]

Closes #139958

The current documentation for std::fs::remove_dir_all function does not explicitly mention the error types that may be returned in concurrent scenarios. Specifically, when one thread attempts to remove a directory tree while another thread simultaneously writes files to that directory, the function may return an io::ErrorKind::DirectoryNotEmpty error, but this behavior is not clearly mentioned in the current documentation.

r? libs