Code.get_docs/2: check for valid doc kind in public function, not in  private
Code.get_docs/2: check for valid doc kind in public function, not in  private

This was giving me a long cryptic message because we were checking for a valid kind
way to far.

I have renamed the attribute to @doc_kinds instead of @doc_sections

Corrected kinds for :all option

Mention that no valid module returns nil, and created a doctest example.

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