Document the wipe features of Disks

Disks in Jessie now advertise a wipe feature. When you create a new partition it proposes to “overwrite exisiting data with zeroes”.

We should investigate how does this work and how does it do compared to “wipe available disk space” in relation with #7118 (closed).

Original created by @sajolida on 8047 (Redmine)

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