kasan: update documentation

kasan: update documentation

This patch updates KASAN documentation to reflect the addition of the new
tag-based mode.

Link: http://lkml.kernel.org/r/1ace22e3a154ce363661bda6328f8c5eb05a091c.1543337629.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>