docs: self-protection: rename "leak" to "exposure"

The meaning of "leak" can be both "untracked resource allocation" and
"memory content disclosure". This document's use was entirely of the
latter meaning, so avoid the confusion by using the Common Weakness
Enumeration name for this: Information Exposure (CWE-200). Additionally
adds a section on structure randomization.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>