Improve GenServer docs
Improve GenServer docs

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