[Lut HAL] update documentation

  - make it clear about how to set `offsets` in the Luts

Bug: 380894872
Test: builds
Change-Id: I91be612caefe3517fabb5420671fee723ba5f9c1
diff --git a/graphics/composer/aidl/android/hardware/graphics/composer3/Luts.aidl b/graphics/composer/aidl/android/hardware/graphics/composer3/Luts.aidl
index 393354e..6ff1db2 100644
--- a/graphics/composer/aidl/android/hardware/graphics/composer3/Luts.aidl
+++ b/graphics/composer/aidl/android/hardware/graphics/composer3/Luts.aidl
@@ -58,6 +58,8 @@
      *
      * Multiple Luts can be packed into one same `pfd`, and `offsets` is used to pinpoint
      * the starting point of each Lut.
+     *
+     * `offsets` should be valid unless an invalid `pfd` is provided.
      */
     @nullable int[] offsets;