kfence, Documentation: add KFENCE documentation

Add KFENCE documentation in dev-tools/kfence.rst, and add to index.

[elver@google.com: add missing copyright header to documentation]
  Link: https://lkml.kernel.org/r/20210118092159.145934-4-elver@google.com

Link: https://lkml.kernel.org/r/20201103175841.3495947-8-elver@google.comSigned-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Co-developed-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Jann Horn <jannh@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christopher Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hillf Danton <hdanton@sina.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joern Engel <joern@purestorage.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: SeongJae Park <sjpark@amazon.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/index.rst

@@ -22,6 +22,7 @@ whole; patches welcome!
    ubsan
    kmemleak
    kcsan
+   kfence
    gdb-kernel-debugging
    kgdb
    kselftest

lib/Kconfig.kfence

@@ -13,6 +13,8 @@ menuconfig KFENCE
 	  to have negligible cost to permit enabling it in production
 	  environments.
 
+	  See <file:Documentation/dev-tools/kfence.rst> for more details.
+
 	  Note that, KFENCE is not a substitute for explicit testing with tools
 	  such as KASAN. KFENCE can detect a subset of bugs that KASAN can
 	  detect, albeit at very different performance profiles. If you can