Document context options for Scene and Viewer

[jjhembd]

The Cesium.Scene constructor inputs an options object which includes a contextOptions property. This property points to an object with many nested properties, a few of which are hand-documented in the main docstring for Scene. Some options such as options.contextOptions.requestWebgl2 are not documented at all.

It would be better to document the entire contextOptions object.

This also affects Cesium.Viewer. The constructor for Viewer has a options.contextOptions parameter which is passed along to Scene.