Documentation shouldn't call things functions that aren't functions

Referring to non-function values (particularly of type IO) as "functions" is often a sign that beginners haven't fully grasped some concepts about purity, so I think it's important to clean up our own language.

For example, in the Prelude docs we have a section called "Input functions" in which only one of the four values is a function:

getChar :: IO Char
getLine :: IO String
getContents :: IO String
interact :: (String -> String) -> IO ()

In that case, I suggest we rename the sections "Input functions" and "Output functions" to simply "Input" and "Output".