add "available since version" information to warnings documentation
Summary
Location of documentation issue: the GHC user's guide
Currently many of the warnings listed in https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/using-warnings.html are missing information about which ghc version they were added in.
This report was prompted by https://github.com/kowainik/summoner/pull/447
Proposed improvements or changes
In partcular I reported there that
-Wcompat
-Widentities
-Wincomplete-uni-patterns
-Wincomplete-record-updates
are only available in ghc8 (well not in ghc-7.10.3 but they are in 8.0.2 (and I assume also 8.0.1 but didn't verify). Anyway since it confused the Summoner maintainers, it would be good to document these anyway and I can open a MR but it would be nice to document them all. I am not sure if there is a easy way to script getting this info from the git source.