lib: add note about process exit message for DEBUG_STACK_USAGE

lib: add note about process exit message for DEBUG_STACK_USAGE

DEBUG_STACK_USAGE doesn't only have an influence on the output of sysrq-T
and sysrq-P, it also enables a message at process exit.  See
check_stack_usage() in kernel/exit.c where this is implemented.

Link: https://lkml.kernel.org/r/20231219182808.210284-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Kees Cook <keescook@chromium.org>
Cc: Marco Elver <elver@google.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>