Warn against file wiping shortcomings

Our current secure deletion documentation does not mention simply wiping a file might not be enough. It probably should.

E.g. "deleted" copies of a file (e.g. with OpenOffice.org). The nautilus-wipe doc probably has more examples about it.

Feature Branch: doc/5831-wipe_shortcomings

Original created by @tails on 5831 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information