Clear 'to do' and fill gaps in specification history >1.12 && <1.18

[mpilgrem]

There is a 'to do' at https://cabal.readthedocs.io/en/latest/file-format-changelog.html#cabal-version-1-16. The 'to do' is likely because changes to Cabal were not logged after Cabal-1.10.2.0 until Cabal-1.18.1.7 and have to be identified by examining the source code of package versions.

I'll raise a pull request that seeks to fill the gaps and eliminate the 'to do'.