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/[e-mail]
Signed-off-by: Uwe Kleine-König <[e-mail]>
Cc: Douglas Anderson <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Marco Elver <[e-mail]>
Cc: "Paul E. McKenney" <[e-mail]>
Cc: Pengutronix Kernel Team <[e-mail]>
Cc: Petr Mladek <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Stephen Boyd <[e-mail]>
Cc: Zhaoyang Huang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/Kconfig.debug

···
 	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.