serial/sysrq: Add a help-string for MAGIC_SYSRQ_SERIAL_SEQUENCE

To make it more obvious what almost everyone wants to set here.

Cc: Iurii Zaikin <yzaikin@google.com>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Joe Perches <joe@perches.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Vasiliy Khoruzhick <vasilykh@arista.com>
Cc: linux-serial@vger.kernel.org
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20200306153156.579921-1-dima@arista.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/Kconfig.debug

@@ -439,6 +439,8 @@ config MAGIC_SYSRQ_SERIAL_SEQUENCE
 	  Specifies a sequence of characters that can follow BREAK to enable
 	  SysRq on a serial console.
 
+	  If unsure, leave an empty string and the option will not be enabled.
+
 config DEBUG_FS
 	bool "Debug Filesystem"
 	help