Lingering @since TODO Haddock annotations

There are currently a handful of @since TODO annotations in Haddocks throughout the core libraries that were probably intended to be replaced with a real version at some point, but managed to slip through the cracks into the GHC 8.4.1 release:

• FixIOException in base: http://git.haskell.org/ghc.git/blob/270e3e9bbaabad3d9a1348cea9e46a9ecf1e5ec2:/libraries/base/GHC/IO/Exception.hs#l276
• powModSecInteger in integer-gmp: http://git.haskell.org/ghc.git/blob/270e3e9bbaabad3d9a1348cea9e46a9ecf1e5ec2:/libraries/integer-gmp/src/GHC/Integer/Type.hs#l1454

We should fix these before GHC 8.4.2.

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	libraries (other)
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