KHR_materials_volume extension doesn't state intended color spaces for properties

While looking into a few color management problems I've come across at least one extension where the intended color space is not specified.

These readmes explicitly mention intended color space:
- https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_lights_punctual/README.md
- light color is linear
- https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/Specification.adoc#392-metallic-roughness-material
- baseColorFactor and baseColor are sRGB
- emissiveFactor and emissive are sRGB (which I think has been discussed as problematic a number of times)
- vertex color is linear
- everything else is linear
- https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_clearcoat/README.md
- everything is linear

These don't:
- https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_volume/README.md#properties
- thicknessTexture is likely intended as linear data
- attenuationColor is likely intended as sRGB (?)

Read extensions/2.0/Khronos/KHR_materials_volume/README.md#properties and compare its property descriptions with the referenced KHR_lights_punctual, Specification.adoc, and KHR_materials_clearcoat documentation. Confirm the intended color space for each relevant volume property with the project’s existing guidance, then update the extension documentation so those color spaces are stated consistently.

Written by the indexing model from the issue text.