add comment that HDR-focused encodings do not support transparency

src/Three.js

@@ -310,9 +310,12 @@ THREE.TriangleFanDrawMode = 2;
 
 THREE.LinearEncoding = 3000; // No encoding at all.
 THREE.sRGBEncoding = 3001;
-THREE.RGBEEncoding = 3002; // AKA Radiance
+THREE.GammaEncoding = 3007; // uses GAMMA_FACTOR, for backwards compatibility with WebGLRenderer.gammaInput/gammaOutput
+
+// The following Texture Encodings are for RGB-only (no alpha) HDR light emission sources.
+// These encodings should not specified as output encodings except in rare situations.
+THREE.RGBEEncoding = 3002; // AKA Radiance.
 THREE.LogLuvEncoding = 3003;
 THREE.RGBM7Encoding = 3004;
 THREE.RGBM16Encoding = 3005;
-THREE.RGBDEncoding = 3006; // MaxRange is 256
-THREE.GammaEncoding = 3007; // uses GAMMA_FACTOR
+THREE.RGBDEncoding = 3006; // MaxRange is 256.