docs: Add documents for openflow actions

lib/openflow/actions/copy_ttl_out.ex

@@ -1,18 +1,38 @@
 defmodule Openflow.Action.CopyTtlOut do
+  @moduledoc """
+  Copy the TTL from outermost to next-to-outermost header with TTL.\\
+  Copy can be IP-to-IP, MPLS-to-MPLS, or MPLS-to-IP.
+  """
+
   defstruct([])
 
   alias __MODULE__
 
+  @type t :: %CopyTtlOut{}
+
+  @spec ofpat() :: 11
   def ofpat, do: 11
 
+  @doc """
+  Create a copy_ttl_out action structure
+
+  ## Example
+
+  ```elixir
+  iex> %CopyTtlOut{} = Openflow.Action.CopyTtlOut.new()
+  ```
+  """
+  @spec new() :: CopyTtlOut.t()
   def new do
     %CopyTtlOut{}
   end
 
+  @spec to_binary(CopyTtlIn.t()) :: <<_::16, _::_*8>>
   def to_binary(%CopyTtlOut{}) do
     <<11::16, 8::16, 0::size(4)-unit(8)>>
   end
 
+  @spec read(<<_::16, _::_*8>>) :: CopyTtlOut.t()
   def read(<<11::16, 8::16, _::size(4)-unit(8)>>) do
     %CopyTtlOut{}
   end