#590 add information about @spec annotation in documentation lesson (#676)
#590 add information about @spec annotation in documentation lesson (#676)

* #590 add information about @spec annotation in documentation lesson

* Small wording changes

huydx/elixir-school,doomspork/elixir_school,psantos10/elixir-school,ElixirVietnam/elixir-school,ElixirVietnam/elixir-school,nscyclone/elixir-school,huydx/elixir-school,huydx/elixir-school,psantos10/elixir-school,doomspork/elixir_school,nscyclone/elixir-school,erickgnavar/elixir-school,ElixirVietnam/elixir-school,yuriibodarev/elixir-school,nscyclone/elixir-school,yuriibodarev/elixir-school,erickgnavar/elixir-school,doomspork/elixir-school,doomspork/elixir-school,doomspork/elixir_school,erickgnavar/elixir-school,doomspork/elixir-school,nscyclone/elixir-school,psantos10/elixir-school,yuriibodarev/elixir-school