docs: deprecated.rst: Clean up fall-through details

Add example of fall-through, list-ify the case ending statements, and
adjust the markup for links and readability. While here, adjust
strscpy() details to mention strscpy_pad().

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/202003041102.47A4E4B62@keescook


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