Cleanup: GPU: Update classes comments

This should avoid confusion about what is a class and what is an opaque
pointer.

source/blender/gpu/opengl/gl_shader_interface.hh

@@ -40,6 +40,9 @@ namespace blender::gpu {
 
 class GLVaoCache;
 
+/**
+ * Implementation of Shader interface using OpenGL.
+ **/
 class GLShaderInterface : public ShaderInterface {
  private:
   /** Reference to VaoCaches using this interface */