docs: Add documents for openflow actions

lib/openflow/actions/push_mpls.ex

@@ -1,20 +1,43 @@
 defmodule Openflow.Action.PushMpls do
+  @moduledoc """
+  Push a new MPLS label
+
+      send_flow_mod_add(
+        datapath_id,
+        match: Match.new(),
+        instruction: ApplyActions.new(PushMpls.new())
+      )
+  """
+
   defstruct(ethertype: 0x8847)
 
   @eth_p_mpls_uc 0x8847
 
   alias __MODULE__
 
+  @type t :: %PushMpls{ethertype: 0..0xFFFF}
+
+  @spec ofpat() :: 19
   def ofpat, do: 19
 
+  @doc """
+  Create a new push_mpls action struct\\
+  0x8847(ETH_P_MPLS_UC) as a default value.
+
+      iex> %PushMpls{ethertype: 0x8847} = PushMpls.new()
+  """
+  @spec new() :: t()
+  @spec new(ethertype :: 0..0xFFFF) :: t()
   def new(ethertype \\ @eth_p_mpls_uc) do
     %PushMpls{ethertype: ethertype}
   end
 
+  @spec to_binary(t()) :: <<_::16, _::_*8>>
   def to_binary(%PushMpls{ethertype: ethertype}) do
     <<19::16, 8::16, ethertype::16, 0::size(2)-unit(8)>>
   end
 
+  @spec read(<<_::16, _::_*8>>) :: t()
   def read(<<19::16, 8::16, ethertype::16, 0::size(2)-unit(8)>>) do
     %PushMpls{ethertype: ethertype}
   end