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>