docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage

docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage

There wasn't any mention of when/where DECLARE_FLEX_ARRAY() should be
used, so add the rationale and an example to the deprecation docs.

Suggested-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230106200600.never.735-kees@kernel.org
[jc: minor wording tweaks]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>