Clarify documentation for the dimming stage.

Plumbing this enum to RenderEngine requires knowledge of the intended
transfer function to apply the dimming stage in. Because this is
expected to be a contrained use-case and because apis are frozen,
document that RenderEngine is allowed to assume that the resulting
dimming matrix may be gamma corrected using a 2.2 power function.

Bug: 218954037
Test: builds
Change-Id: Ie7d357f8ce79295af017d80c62a2759dbccce5d2

graphics/composer/aidl/android/hardware/graphics/composer3/DimmingStage.aidl

@@ -29,7 +29,12 @@ enum DimmingStage {
      */
     LINEAR = 1,
     /**
-     * Dimming operations must be applied in gamma space, after OETF has been applied
+     * Dimming operations must be applied in gamma space, after OETF has been applied.
+     * Note that for this dimming operation to be perceptually correct it must also be gamma
+     * corrected. The framework will assume that it is able to use the gamma 2.2
+     * power function for gamma correcting the dimming matrix, for simplicity of
+     * implementation and the fact that gamma 2.2 is close enough to typical SDR
+     * transfer functions that would be used for the client target.
      */
     GAMMA_OETF = 2,
 }