docs: `loading_message` doc, attributes, and story

lib/medical_transcription_web/components/components.ex

@@ -48,6 +48,11 @@ defmodule MedicalTranscriptionWeb.Components do
     """
   end
 
+  attr :visible, :boolean, required: true
+
+  @doc """
+  A loading icon and message displayed while the audio is being processed.
+  """
   def loading_message(assigns) do
     ~H"""
     <%= if @visible do %>

storybook/components/loading_message.story.exs

@@ -0,0 +1,16 @@
+defmodule MedicalTranscriptionWeb.Storybook.LoadingMessage do
+  use PhoenixStorybook.Story, :component
+
+  def function, do: &MedicalTranscriptionWeb.Components.loading_message/1
+
+  def variations do
+    [
+      %Variation{
+        id: :default,
+        attributes: %{
+          visible: true
+        }
+      }
+    ]
+  end
+end