SL-19564 Document magic number.

indra/newview/app_settings/shaders/class1/deferred/luminanceF.glsl

@@ -44,7 +44,7 @@ float lum(vec3 col)
 void main() 
 {
     vec2 tc = vary_fragcoord*0.6+0.2;
-    tc.y -= 0.1;
+    tc.y -= 0.1; // HACK - nudge exposure sample down a little bit to favor ground over sky
     vec3 c = texture(diffuseRect, tc).rgb + texture(emissiveRect, tc).rgb;
     float L = lum(c);
     frag_color = vec4(L);