`UrlTemplateRasterOverlay`: Incorrect documentation of `{y}` and `{reverseY}`.

[kring]

{y} is numbered from the South, {reverseY} is numbered from the North, but the documentation says the opposite. The doc was probably copied from CesiumJS, which internally numbers tiles from the North.

This doc changes also needs to be propagated to Unreal and Unity:

• https://github.com/CesiumGS/cesium-unreal/blob/v2.16.0/Source/CesiumRuntime/Public/CesiumUrlTemplateRasterOverlay.h#L43
• https://github.com/CesiumGS/cesium-unity/blob/v1.16.0/Runtime/CesiumUrlTemplateRasterOverlay.cs#L42

CC CesiumGS/cesium-unity#573