DT: leds: Improve examples by adding some context

During my work on some new LED trigger I tried adding example similar to
the existing ones which received following comment from Rob:
> It's not really clear in the example this is an LED node as it is
> incomplete.

Keeping that in mind I suggest adding context for the existing example
entries in hope to make documentation more clear.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>