For this and `czm_getSpecular`, we generally add full doc when we add new functions even if they previously existed and were not documented.  Actually, we often add doc to a function if it didn't already have it, even if we just modify the function.

[eyeToWindowCoordinates.glsl](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Shaders/Builtin/Functions/eyeToWindowCoordinates.glsl) is a reasonable example of how to doc a GLSL function.

These should also be mentioned in CHANGES.md.