docs: lkdtm: Modernize and improve details

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>