docs: Add documents for openflow actions

lib/openflow/actions/copy_ttl_in.ex

@@ -1,18 +1,37 @@
 defmodule Openflow.Action.CopyTtlIn do
+  @moduledoc """
+  Copy the TTL from next-to-outermost to outermost header with TTL.\\
+  Copy can be IP-to-IP, MPLS-to-MPLS, or IP-to-MPLS.
+  """
+
   defstruct([])
 
   alias __MODULE__
 
+  @type t :: %CopyTtlIn{}
+
+  @spec ofpat() :: 12
   def ofpat, do: 12
 
+  @doc """
+  Create a copy_ttl_in action struct
+
+  ## Example:
+  ```elixir
+  iex> %CopyTtlIn{} = Openflow.Action.CopyTtlIn.new()
+  ```
+  """
+  @spec new() :: CopyTtlIn.t()
   def new do
     %CopyTtlIn{}
   end
 
+  @spec to_binary(CopyTtlIn.t()) :: <<_::16, _::_*8>>
   def to_binary(%CopyTtlIn{}) do
     <<12::16, 8::16, 0::size(4)-unit(8)>>
   end
 
+  @spec read(<<_::16, _::_*8>>) :: CopyTtlIn.t()
   def read(<<12::16, 8::16, _::size(4)-unit(8)>>) do
     %CopyTtlIn{}
   end