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 <[e-mail]>
Signed-off-by: Mans Rullgard <[e-mail]>
[Added a bit more text to address Linus' suggestion]
Signed-off-by: Miguel Ojeda <[e-mail]>

drivers/auxdisplay/Kconfig

···
 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"