kcov: load acquire coverage count in user-space code

Update the KCOV documentation to use a load-acquire operation for the
first element of the shared memory buffer between kernel-space and
user-space.

The load-acquire pairs with the write memory barrier used in
kcov_move_area().

[soham.bagchi@utah.edu: v2]
  Link: https://lkml.kernel.org/r/20250803180558.2967962-1-soham.bagchi@utah.edu
Link: https://lkml.kernel.org/r/20250728184318.1839137-2-soham.bagchi@utah.edu


Signed-off-by: Soham Bagchi <soham.bagchi@utah.edu>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>