hardening: Clarify Kconfig text for auto-var-init

Clarify the details around the automatic variable initialization modes
available. Specifically this details the values used for pattern init
and expands on the rationale for zero init safety. Additionally makes
zero init the default when available.

Cc: glider@google.com
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: linux-security-module@vger.kernel.org
Cc: clang-built-linux@googlegroups.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>