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>

Documentation/dev-tools/kasan.rst

@@ -110,7 +110,9 @@ parameter can be used to control panic and reporting behaviour:
 - ``kasan.fault=report``, ``=panic``, or ``=panic_on_write`` controls whether
   to only print a KASAN report, panic the kernel, or panic the kernel on
   invalid writes only (default: ``report``). The panic happens even if
-  ``kasan_multi_shot`` is enabled.
+  ``kasan_multi_shot`` is enabled. Note that when using asynchronous mode of
+  Hardware Tag-Based KASAN, ``kasan.fault=panic_on_write`` always panics on
+  asynchronously checked accesses (including reads).
 
 Software and Hardware Tag-Based KASAN modes (see the section about various
 modes below) support altering stack trace collection behavior: