Extension compatibility & provisional

So the styleguide says an extension is always fully compatible (except historical accidents). So extensions cannot ever add commands to themselves.

But `VkExtensionProperties::specVersion` says they are backwards-compatible.

And "Provisional" entry says those may break backwards compatibility.

1) That info should be unified.
2) I am not clear on NVX. I think it used to mean (as KHX) the same as provisional? Should NVX extensions be marked provisional? (in separate #808 as suggested)