kasan, doc: note kasan.fault=panic_on_write behaviour for async modes

Note the behaviour of kasan.fault=panic_on_write for async modes, since
all asynchronous faults will result in panic (even if they are reads).

Link: https://lkml.kernel.org/r/ZJHfL6vavKUZ3Yd8@elver.google.com
Fixes: 452c03fd

 ("kasan: add support for kasan.fault=panic_on_write")
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Aleksandr Nogikh <nogikh@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Taras Madan <tarasmadan@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>