KHR_materials_volume: clarify that the calculation of the attenuation coefficient uses a natural logarithm

[expenses]

From https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_volume#attenuation:

The infinite range of the coefficient makes it rather unintuitive to control by users. To provide a convenient parameterization, this extension exposes two derived parameters: attenuation color c and attenuation distance d (see Properties). The relation between the two parameters and the attenuation coefficient σt is defined as

σt = -log(c) / d

I was just reading over these docs and had to double-check with the glTF Sample Viewer code and my own previous implementation that this coefficient uses a natural (base-e) log and not a base-10 log. It'd be great to clarify this as https://en.wikipedia.org/wiki/Attenuation_coefficient is not helpful in this regard.