Clarified method limitations

src/renderers/shaders/ShaderChunk/common.glsl.js

@@ -63,9 +63,11 @@ vec3 transformDirection( in vec3 dir, in mat4 matrix ) {
 
 }
 
-// http://en.wikibooks.org/wiki/GLSL_Programming/Applying_Matrix_Transformations
 vec3 inverseTransformDirection( in vec3 dir, in mat4 matrix ) {
 
+	// dir can be either a direction vector or a normal vector
+	// upper-left 3x3 of matrix is assumed to be orthogonal
+
 	return normalize( ( vec4( dir, 0.0 ) * matrix ).xyz );
 
 }