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/discogs.ex

@@ -1,12 +1,14 @@
 defmodule Discogs do
   alias Discogs.API
 
+  @spec get_artist(integer() | String.t()) :: {:ok, map()} | {:error, term()}
   def get_artist(id) do
     discogs_config = discogs_config()
 
     API.get_artist(id, discogs_config)
   end
 
+  @spec get_artist_image(String.t()) :: {:ok, binary()} | {:error, :cover_not_available}
   def get_artist_image(url) do
     discogs_config = discogs_config()