Add "since" information to LANGUAGE extensions in GHC user guide
If one wants to support older versions of GHC, it is hard to find out whether one can use a certain LANGUAGE extension. One has to go to older versions of the GHC manual to find out whether the corresponding documentation was already there, or browse release notes. Requests:
- Document for new LANGUAGE extensions that they are available since version 7.10. For example, by adding "(Since GHC 7.10)" to the heading of the new feature.
- Do this also for the previous LANGUAGE extensions, by adding e.g., "(Since GHC 7.6)" etc.