Correct FixIOException's @since annotation retroactively

Summary:
In D4113, a `FixIOException` data type was added with a
`@since TODO` annotation, but it seems that `TODO` made it out into
`base-4.11` itself. I've (retroactively) fixed this and added an
entry to the `base-4.11` entry in the `changelog`.

Test Plan: Read it

Reviewers: dfeuer, hvr, bgamari

Reviewed By: dfeuer

Subscribers: thomie, carter

GHC Trac Issues: #14356, #15025

Differential Revision: https://phabricator.haskell.org/D4578

(cherry picked from commit ce27c7d5)