Further improvements to Mix.Config docs
Further improvements to Mix.Config docs

elixir-lang/elixir,pedrosnk/elixir,kelvinst/elixir,joshprice/elixir,kelvinst/elixir,kimshrier/elixir,ggcampinho/elixir,michalmuskala/elixir,ggcampinho/elixir,pedrosnk/elixir,kimshrier/elixir,lexmag/elixir,lexmag/elixir