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/import.ex

@@ -1,4 +1,5 @@
 defmodule LastFm.Import do
+  @spec batch(keyword()) :: {:ok, non_neg_integer()} | {:error, term()}
   def batch(opts) do
     with {:ok, tracks} <- LastFm.get_tracks(opts) do
       LastFm.Feed.update(tracks)