DirectoryInfo.Exists remarks should mention similar warning as FileInfo.Exists

`FileInfo.Exists` mentions that on creation, the `FileInfo` instance will check if the file exists or not. Any additional manipulations require calling `Refresh` beforehand, to ensure we acquire the latest information on the file. For example, to check if the file has been deleted or renamed by an external process.

We need a similar remark in `DirectoryInfo.Exists` because the behavior is expected to be similar.