Various documentation improvements

 * Fix missing code example in changelog for 8.4.1

 * List 'setEnv' as opposite of 'getEnv'

   It seems best to direct users to use 'System.Environment.setEnv'
   rather than 'System.Posix.Env.putEnv'. This is due to 'setEnv' being
   located in the same module as 'getEnv' and my virtue of working on
   Windows platform, whereas 'putEnv' does not have that quality because
   it's part of the 'unix' package.

 * Reflect in docs the fact that 'readMVar' is not a composition of
   'takeMVVar' and 'putMVar' any more