The details on using LKDTM were overly obscure. Modernize the details
and expand examples to better illustrate how to use the interfaces.
Additionally add missing SPDX header.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201015224559.2137489-1-keescook@chromium.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>