Clarify use of PointSelector with DirectionalLight

[JulieWinchester]

The model description for DirectionalLight is:

Directional Lights emit their light in a specific direction as if infinitely far away, and as such the light does not come from a specific position. The rays produced are all parallel.

The light is emitted in the negative Y direction by default, thus straight down, but the orientation of the light can be altered with lookAt or with a Rotate Transform.

For DirectionalLight to have lookAt, there must be some assumed position from which to calculate the orientation for lookAt. (Right?) Although the light doesn't emit from a position, it might be worth clarifying how PointSelector can be used to modify the position from which lookAt should be calculated.

[JulieWinchester]

Speaking of lookAt, the description for lookAt says "If this property is not specified, then the default value for cameras is to look straight backwards (-Z) and for lights to point straight down (-Y)." This should probably be clarified to indicate that this is only the default is transforms aren't also being used.