docs: deprecated.rst: Update zero-length/one-element arrays section

Update information in the zero-length and one-element arrays section
and illustrate how to make use of the new flex_array_size() helper,
together with struct_size() and a flexible-array member.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200901010949.GA21398@embeddedor


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