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/last_fm/album.ex

@@ -13,6 +13,7 @@ defmodule LastFm.Album do
     field :title, :string
   end
 
+  @spec changeset(t(), map()) :: Ecto.Changeset.t()
   def changeset(album, attrs) do
     album
     |> cast(attrs, [:musicbrainz_id, :title])