kasan: docs: update GENERIC implementation details section

Update the "Implementation details" section for generic KASAN:

 - Don't mention kmemcheck, it's not present in the kernel anymore.

 - Don't mention GCC as the only supported compiler.

 - Update kasan_mem_to_shadow() definition to match actual code.

 - Punctuation, readability, and other minor clean-ups.

Link: https://lkml.kernel.org/r/f2f35fdab701f8c709f63d328f98aec2982c8acc.1615559068.git.andreyknvl@google.com


Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>