Replace mentions to string() type with charlist()

[eksperimental]

lib/ex_doc/language/elixir.ex: The function call on line 95 is expected to have type nonempty_list(A) but it has type string()
94   defp doc?({{_, name, _}, _, _, :none, _}, _type) do
95     hd(Atom.to_charlist(name)) != ?_
96   end

Not sure if should be for Erlang functions, (maybe), but certainly for Elixir functions as string() is discouraged.