docs: deprecated.rst: Add %p to the list

Once in a while %p usage comes up, and I've needed to have a reference
to point people to. Add %p details to deprecated.rst.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/202003042301.F844A8C0EC@keescook


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