Secure deletion documentation needs to be updated

it seems that https://tails.boum.org/doc/encryption_and_privacy/secure_deletion/#clean_disk_space step-by-step is not working anymore since a few monthes (maybe since Tails-wheezy). it doesn’t describe correctly the behavior of Nautilus-wipe and doesn’t advertise the user that an administrator password is now needed.

Feature Branch: emmapeel:bugfix/10791-secure_deletion

Original created by @goupille on 10791 (Redmine)

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