GL: forgot to document ImageAccess enum values.

AND DOXYGEN DID NOT TELL ME!! For over eight years! It was undcoumented
since the original d9b25c6057 and I did
not know!!

src/Magnum/GL/ImageFormat.h

@@ -48,8 +48,13 @@ namespace Magnum { namespace GL {
 @requires_gles Shader image load/store is not available in WebGL.
 */
 enum class ImageAccess: GLenum {
+    /** Access the image for reading only */
     ReadOnly = GL_READ_ONLY,
+
+    /** Access the image for writing only */
     WriteOnly = GL_WRITE_ONLY,
+
+    /** Access the image for both reading and writing */
     ReadWrite = GL_READ_WRITE
 };