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.

Author: eksperimental

lib/elixir/lib/code.ex

@@ -588,7 +588,10 @@ defmodule Code do
     * `:type_docs` - list of all docstrings attached to
       `@type` callbacks using the `@typedoc` attribute
 
-    * `:all` - a keyword list with both `:docs` and `:moduledoc`
+    * `:all` - a keyword list with `:docs` and `:moduledoc`, `:callback_docs`,
+      and `:type_docs`.
+
+  If the module cannot be found, it returns `nil`.
 
   ## Examples
 
@@ -598,8 +601,14 @@ defmodule Code do
       iex> String.split(text, "\n") |> Enum.at(0)
       "Converts an atom to a char list."
 
+      # Module doesn't exist
+      iex> docs = Code.get_docs(ModuleNotGood, :all)
+      nil
+
   """
-  def get_docs(module, kind) when is_atom(module) do
+  @doc_kinds [:docs, :moduledoc, :callback_docs, :type_docs, :all]
+
+  def get_docs(module, kind) when is_atom(module) and kind in @doc_kinds do
     case :code.get_object_code(module) do
       {_module, bin, _beam_path} ->
         do_get_docs(bin, kind)
@@ -608,7 +617,7 @@ defmodule Code do
     end
   end
 
-  def get_docs(binpath, kind) when is_binary(binpath) do
+  def get_docs(binpath, kind) when is_binary(binpath) and kind in @doc_kinds do
     do_get_docs(String.to_char_list(binpath), kind)
   end
 
@@ -629,10 +638,8 @@ defmodule Code do
   # unsupported chunk version
   defp lookup_docs(_, _), do: nil
 
-  @doc_sections [:docs, :moduledoc, :callback_docs, :type_docs]
-
   defp do_lookup_docs(docs, :all), do: docs
-  defp do_lookup_docs(docs, kind) when kind in @doc_sections,
+  defp do_lookup_docs(docs, kind),
     do: Keyword.get(docs, kind)
 
   ## Helpers