Update howto-manually-remove-assets documentation

[floehopper]

It looks as if this page is possibly slightly out-of-date, because it talks about deleting the file from the NFS mount, but now the asset is also stored on S3 and will not be deleted even when Asset#destroy is invoked. We may need to introduce a way to do remove the object from S3 if desired, but it would be worth understanding the motivation behind wanting to do this.