docs(set_leds): note that segment start/length are now optional

The set_leds service handler is already pass-through, so the new
server-side defaults (start=0, length=rest-of-strip) work with no
code change. Refresh the field descriptions in services.yaml and
strings.json so the UI surfaces the simpler {"mode","color"} payload.

custom_components/ledgrab/services.yaml

@@ -11,9 +11,11 @@ set_leds:
     segments:
       name: Segments
       description: >
-        List of segment objects. Each segment has: start (int), length (int),
-        mode ("solid"/"per_pixel"/"gradient"), color ([R,G,B] for solid),
-        colors ([[R,G,B],...] for per_pixel/gradient)
+        List of segment objects. Each segment has: start (int, optional —
+        defaults to 0), length (int, optional — defaults to the rest of the
+        strip from start), mode ("solid"/"per_pixel"/"gradient"), color
+        ([R,G,B] for solid), colors ([[R,G,B],...] for per_pixel/gradient).
+        Example fill-whole-strip: [{"mode": "solid", "color": [255, 0, 0]}].
       required: true
       selector:
         object: