717: Fixup WebGL Building Docs r=kvark a=cwfitzgerald Improved the docs for building wgpu for targeting webgl. This would have prevented https://github.com/gfx-rs/gfx/issues/3594. I also think (but haven't changed yet) that all the examples should use `wgpu::BackendBit::all()` as that is really what people should be defaulting to. (Rendered: https://github.com/cwfitzgerald/wgpu-rs/blob/webgl-docs/README.md#run-examples-on-the-web-wasm32-unknown-unknown) Co-authored-by: Connor Fitzgerald <connorwadefitzgerald@gmail.com>