update doc in pixelgl
// Package pixelgl provides some abstractions around the basic OpenGL primitives and
// operations.
//
-// All calls should be wrapped inside pixelgl.Do/DoNoBlock/DoErr/DoVal.
+// All calls should be done from the main thread using "github.com/faiface/mainthread" package.
//
// This package deliberately does not handle nor report OpenGL errors, it's up to you to
// cause none.
