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