Add docs for change_playback_state

lib/membrane/core/playback.ex

@@ -22,4 +22,6 @@ defmodule Membrane.Core.Playback do
         }
 
   @type state_t :: :stopped | :prepared | :playing
+
+  defguard is_playback_state(atom) when atom in [:stopped, :prepared, :playing]
 end

lib/membrane/core/playback_requestor.ex

@@ -10,27 +10,39 @@ defmodule Membrane.Core.PlaybackRequestor do
   defmacro __using__(_args) do
     quote location: :keep do
       @behaviour unquote(__MODULE__)
+      require Membrane.Core.Playback
 
       @doc """
       Changes playback state to `:playing`.
+
+      An alias for `change_playback_state/2` with proper state.
       """
       @spec play(pid) :: :ok
       def play(pid), do: change_playback_state(pid, :playing)
 
       @doc """
       Changes playback state to `:prepared`.
+
+      An alias for `change_playback_state/2` with proper state.
       """
       @spec prepare(pid) :: :ok
       def prepare(pid), do: change_playback_state(pid, :prepared)
 
       @doc """
       Changes playback state to `:stopped`.
+
+      An alias for `change_playback_state/2` with proper state.
       """
       @spec stop(pid) :: :ok
       def stop(pid), do: change_playback_state(pid, :stopped)
 
       @impl unquote(__MODULE__)
-      def change_playback_state(pid, new_state) do
+      @doc """
+      Changes the playback state to the `new_state`.
+      """
+      @spec change_playback_state(pid, Playback.state_t()) :: :ok
+      def change_playback_state(pid, new_state)
+          when Membrane.Core.Playback.is_playback_state(new_state) do
         alias Membrane.Core.Message
         require Message
         Message.send(pid, :change_playback_state, new_state)