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.
Ah, I see. That is pretty cool. This subtlety is not very obvious from looking at the WebGL1 spec (`WebGLRenderingContextBase` is only mentioned in the IDL). This text may be read by developers interested in using WebGL extensions so I'm not sure the WebGL1/WebGL2 cross-compatibility intention is really expressed here or in either of the specifications.

Thanks for your work!