deprecated.rst: Include details on "no_hash_pointers"

Linus decided a debug toggle for %p was tolerable, so update the
%p deprecation documentation.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210723200526.3424128-1-keescook@chromium.org


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