HDRP: Update comment of integrate HDRI and clean a little code

com.unity.render-pipelines.high-definition/HDRP/Editor/Sky/HDRISky/HDRISkyEditor.cs

 
             float omegaP = (Mathf.PI * 4) / (6.0f * hdri.width * hdri.width);
             m_IntegrateHDRISkyMaterial.SetTexture(HDShaderIDs._Cubemap, hdri);
-            m_IntegrateHDRISkyMaterial.SetFloat(HDShaderIDs._InvOmegaP, 1.0f / omegaP);
 
             Graphics.Blit(Texture2D.whiteTexture, m_IntensityTexture.rt, m_IntegrateHDRISkyMaterial);
 
             base.CommonSkySettingsGUI();
         }
     }
-}
+}

com.unity.render-pipelines.high-definition/HDRP/Sky/HDRISky/IntegrateHDRISky.shader

     {
         [HideInInspector]
         _Cubemap ("", CUBE) = "white" {}
-        _InvOmegaP ("", Float) = 0
     }
 
     SubShader
             };
 
             TextureCube<float4> _Cubemap;
-            float _InvOmegaP;
 
             Varyings Vert(Attributes input)
             {
             }
             
 
-            // Spherical integration of the upper hemisphere
+            // With HDRI that have a large range (including the sun) it can be challenging to
+            // compute the lux value without multiple importance sampling.
+            // We instead use a brute force Uniforme Spherical integration of the upper hemisphere
+            // with a large number of sample. This is fine as this happen in the editor.
             real GetUpperHemisphereLuxValue(TEXTURECUBE_ARGS(skybox, sampler_skybox), real3 N)
             {
                 float sum = 0.0;
                 {
                     for (float theta = 0; theta < PI / 2.0; theta += dtheta)
                     {
+                        // SphericalToCartesian function is for Z up, lets move to Y up with TransformGLtoDX
                         float3 L = TransformGLtoDX(SphericalToCartesian(phi, cos(theta)));
                         real val = Luminance(SAMPLE_TEXTURECUBE_LOD(skybox, sampler_skybox, L, 0).rgb);
                         sum += cos(theta) * sin(theta) * val;
 
             float4 Frag(Varyings input) : SV_Target
             {
+                // Integrate upper hemisphere (Y up)
                 float3 N = float3(0.0, 1.0, 0.0);
 
                 float intensity = GetUpperHemisphereLuxValue(TEXTURECUBE_PARAM(_Cubemap, s_trilinear_clamp_sampler), N);
         }
     }
     Fallback Off
-}
+}