Core-spec is woefully out of date

Our core-spec document (https://gitlab.haskell.org/ghc/ghc/-/blob/master/docs/core-spec/core-spec.pdf) is woefully out of date.

There are at least two problems:

1. Recent refactoring in the linter is not reflected in the document.

2. Linear types is not reflected in the document.

I will fix, in due course.