docs: Add documents for openflow actions

lib/openflow/actions/dec_mpls_ttl.ex

@@ -1,18 +1,39 @@
 defmodule Openflow.Action.DecMplsTtl do
+  @moduledoc """
+  Decrement MPLS TTL action
+  """
+
   defstruct([])
 
   alias __MODULE__
 
+  @type t :: %DecMplsTtl{}
+
+  @spec ofpat() :: 16
   def ofpat, do: 16
 
+  @doc """
+  Create a new dec_mpls_ttl action struct.
+
+  Note: Need to be specified the one of `ETH_P_MPLS_*` on `eth_type` match field when using this action.
+
+  ## Example:
+
+  ````elixir
+  iex> %DecMplsTtl{} = DecMplsTtl.new()
+  ````
+  """
+  @spec new() :: t()
   def new do
     %DecMplsTtl{}
   end
 
+  @spec to_binary(t()) :: <<_::16, _::_*8>>
   def to_binary(%DecMplsTtl{}) do
     <<16::16, 8::16, 0::size(4)-unit(8)>>
   end
 
+  @spec read(<<_::16, _::_*8>>) :: t()
   def read(<<16::16, 8::16, _::size(4)-unit(8)>>) do
     %DecMplsTtl{}
   end