"Since" information for `MonadFail` instances missing (haddock, CHANGELOG)

After doing a bit of research, I conjecture that the `MonadFail` instances (e.g. for `ReaderT`) were added in `mtl-2.2.1`.
However, this information isn't attached to these instances in the haddocks (e.g. https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Reader.html#v:fail), nor is it mentioned in the CHANGELOG (https://hackage.haskell.org/package/mtl-2.2.2/changelog).

Proposal:
- [ ] add missing `since` information to the docs
- [ ] also add to CHANGELOG

Start with the MonadFail instance Haddock linked in the issue, using the ReaderT instance as the example, and inspect the CHANGELOG for the corresponding release history. Confirm when the instances were added, then ensure the Haddock includes the missing since information and the CHANGELOG records it.

Written by the indexing model from the issue text.