docs: Add documents for openflow actions

lib/openflow/actions/pop_vlan.ex

@@ -1,18 +1,38 @@
 defmodule Openflow.Action.PopVlan do
+  @moduledoc """
+  Pop the outer VLAN tag.
+
+  note: The one of ETH__P_802_* is needed to be specified to eth_type field
+      send_flow_mod_add(
+        datapath_id,
+        match: Match.new(eth_type: 0x8100),
+        instructions: ApplyActions.new(PopVlan.new())
+      )
+  """
+
   defstruct([])
 
   alias __MODULE__
 
+  @type t() :: %PopVlan{}
+
+  @spec ofpat() :: 18
   def ofpat, do: 18
 
+  @doc """
+  Create a new pop_vlan action struct
+  """
+  @spec new() :: t()
   def new do
     %PopVlan{}
   end
 
+  @spec to_binary(t()) :: <<_::16, _::_*8>>
   def to_binary(%PopVlan{}) do
     <<18::16, 8::16, 0::size(4)-unit(8)>>
   end
 
+  @spec read(<<_::16, _::_*8>>) :: t()
   def read(<<18::16, 8::16, _::size(4)-unit(8)>>) do
     %PopVlan{}
   end