Fix documentation regarding removed hook variable

Fixes #217

migration.md

@@ -41,6 +41,8 @@ const {
 } = useThree()
 ```
 
+**[DEPRECATED]**: `useThree().canvas`. Replace by using `useThree().gl.domElement`.
+
 #### useFrame(callback: (state, delta) => void, renderPriority: number = 0)
 
 This hooks calls you back every frame, which is good for running effects, updating controls, etc. You receive the state (same as useThree) and a clock delta. If you supply a render priority greater than zero it will switch off automatic rendering entirely, you can then control rendering yourself. If you have multiple frames with a render priority then they are ordered highest priority last, similar to the web's z-index. Frames are managed, three-fiber will remove them automatically when the component that holds them is unmounted.

readme.md

@@ -274,7 +274,6 @@ import { useThree } from 'react-three-fiber'
 
 const {
   gl,                           // WebGL renderer
-  canvas,                       // canvas the dom element that was created
   scene,                        // Default scene
   camera,                       // Default camera
   size,                         // Bounds of the view (which stretches 100% and auto-adjusts)