These comments are rather less useful now that haddock can give docs
with the same informatino in the module synopsis.

Having to maintain them when making changes to the library is a pain,
and when people forget about doing so there is nothing that checks that
the comments are right, so mistakes tend to linger.

Of the comments that my script detected, 78 of 684 were already
incorrect in one way or another, e.g. missing context:
    Text.Show.showsPrec
    Comment type: Int -> a -> ShowS
    Actual type:  Show a => Int -> a -> ShowS
wrong context:
    Numeric.readInt
    Comment type: Integral a => a -> (Char -> Bool) -> (Char -> Int) -> ReadS a
    Actual type:  Num a => a -> (Char -> Bool) -> (Char -> Int) -> ReadS a
not following a class change (e.g. Num losing its Eq superclass):
    Text.Read.Lex.readOctP
    Comment type: Num a => ReadP a
    Actual type:  (Eq a, Num a) => ReadP a
not following the Exceptions change:
    GHC.Conc.childHandler
    Comment type: Exception -> IO ()
    Actual type:  SomeException -> IO ()
or just always been wrong:
    GHC.Stable.deRefStablePtr
    Comment type: StablePtr a -> a
    Actual type:  StablePtr a -> IO a