overflow: Fix kern-doc markup for functions

Fix the kern-doc markings for several of the overflow helpers and move
their location into the core kernel API documentation, where it belongs
(it's not driver-specific).

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: linux-hardening@vger.kernel.org
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>