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.deSigned-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>

lib/Kconfig.debug

@@ -763,6 +763,8 @@ config DEBUG_STACK_USAGE
 	help
 	  Enables the display of the minimum amount of free stack which each
 	  task has ever had available in the sysrq-T and sysrq-P debug output.
+	  Also emits a message to dmesg when a process exits if that process
+	  used more stack space than previously exiting processes.
 
 	  This option will slow down process creation somewhat.