Documentation/features/KASAN: mark KASAN as supported only on 64-bit on x86

Relevant part is:
arch/x86/Kconfig: select HAVE_ARCH_KASAN  if X86_64 && SPARSEMEM_VMEMMAP

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/features/debug/KASAN/arch-support.txt

@@ -34,6 +34,6 @@
     |        tile: | TODO |
     |          um: | TODO |
     |   unicore32: | TODO |
-    |         x86: |  ok  |
+    |         x86: |  ok  | 64-bit only
     |      xtensa: | TODO |
     -----------------------