Documentation: leds: leds-class: Document keyboard backlight LED class naming

Documentation: leds: leds-class: Document keyboard backlight LED class naming

Document the existing practice of always using 'kbd_backlight' for
the function part of LED class device names for LED class devices which
control single-zone keyboard backlights.

Also extend this existing practice with a new naming scheme for keyboards
with zoned backlight control. There are several drivers in the works (see
the Link:tags below) which offer backlight control for keyboards where
the keyboard backlight is divided in a limited number of zones, e.g.
"main", "cursor" and "numpad" zones.

It is important to agree on a consistent naming scheme for these now,
so that userspace can support multiple different models / vendors through
a single unified naming scheme.

Link: https://lore.kernel.org/platform-driver-x86/20230131235027.36304-1-rishitbansal0@gmail.com/
Link: https://lore.kernel.org/platform-driver-x86/20240719100011.16656-1-carlosmiguelferreira.2003@gmail.com/
Link: https://lore.kernel.org/platform-driver-x86/20260304105831.119349-3-edip@medip.dev/
Link: https://lore.kernel.org/platform-driver-x86/20240806205001.191551-2-mustafa.eskieksi@gmail.com/
Link: https://lore.kernel.org/linux-input/20260402075239.3829699-1-xav@bes.tel/
Signed-off-by: Carlos Ferreira <carlosmiguelferreira.2003@gmail.com>
Co-developed-by: Hans de Goede <johannes.goede@oss.qualcomm.com>
Signed-off-by: Hans de Goede <johannes.goede@oss.qualcomm.com>
Acked-by: Kate Hsuan <hpa@redhat.com>
Link: https://patch.msgid.link/20260504145434.12746-1-johannes.goede@oss.qualcomm.com
Signed-off-by: Lee Jones <lee@kernel.org>