auxdisplay: charlcd: add help text for backlight initial state

While the individual CHARLCD_BL_xxx options have help texts, the
menu itself does not.  Fix this.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mans Rullgard <mans@mansr.com>
[Added a bit more text to address Linus' suggestion]
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

drivers/auxdisplay/Kconfig

@@ -448,6 +448,11 @@ config PANEL_BOOT_MESSAGE
 choice
 	prompt "Backlight initial state"
 	default CHARLCD_BL_FLASH
+	---help---
+	  Select the initial backlight state on boot or module load.
+
+	  Previously, there was no option for this: the backlight flashed
+	  briefly on init. Now you can also turn it off/on.
 
 	config CHARLCD_BL_OFF
 		bool "Off"