docs: deprecated.rst: Add zero-length and one-element arrays

Add zero-length and one-element arrays to the list.

While I continue replacing zero-length and one-element arrays with
flexible-array members, I need a reference to point people to, so
they don't introduce more instances of such arrays. And while here,
add a note to the "open-coded arithmetic in allocator arguments"
section, on the use of struct_size() and the arrays-to-deprecate
mentioned here.

Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200608213711.GA22271@embeddedor


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