First pass at uniformed types, specs and docs

- spec public functions (skipping controllers, views, live views and
components)
- use types instead of explanations in docs
- remove redundant docs
- fix typos

lib/music_library_web/duration.ex

@@ -21,6 +21,7 @@ defmodule MusicLibraryWeb.Duration do
       "0:00"
 
   """
+  @spec format_duration(non_neg_integer()) :: String.t()
   def format_duration(milliseconds) do
     milliseconds
     |> System.convert_time_unit(:millisecond, :second)