Clarify the comment

Assets/ScriptableRenderLoop/ShaderLibrary/ImageBasedLighting.hlsl

     return perceptualRoughness * UNITY_SPECCUBE_LOD_STEPS;
 }
 
-// The inverse of perceptualRoughnessToMipmapLevel().
+// The inverse of the *approximated* version of perceptualRoughnessToMipmapLevel().
 float mipmapLevelToPerceptualRoughness(float mipmapLevel)
 {
     float perceptualRoughness = saturate(mipmapLevel / UNITY_SPECCUBE_LOD_STEPS);