Improve docs for System.get_env/0 and System.get_env/1
Improve docs for System.get_env/0 and System.get_env/1

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