Add information on planned backwards incompatibility in WebGL 2

It was decided in the WebGL WG F2F that WebGL 2 will not be entirely
backwards compatible with WebGL 1. In particular, there is agreement
that all extensions are not moving over, and always enabling primitive
restart in WebGL 2 significantly streamlines implementations, so that
could be added to this section of the spec.

Fixes small issues in the extension registry.
It's a NoInterfaceObject which WebGLRenderingContext and WebGLRenderingContext2 inherit from. So the object type is not visible to JS, but it used to show which parts of the API are shared. Basically this change is meant to express that both WebGL 1 and WebGL 2 contexts are extensible.