Hashing addresses printed with printk specifier %p was implemented
recently. During development a number of issues were raised regarding
leaking kernel addresses to userspace. Other documentation was updated but
security/self-protection missed out.

Add self-protection documentation regarding printing kernel addresses.
Signed-off-by: NTobin C. Harding <me@tobin.cc>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>