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_brainz.ex

@@ -1,6 +1,15 @@
 defmodule MusicBrainz do
   alias MusicBrainz.API
 
+  @type search_opts :: [limit: non_neg_integer(), offset: non_neg_integer()]
+
+  @spec search_release_group(String.t(), search_opts()) ::
+          {:ok,
+           %{
+             total_count: non_neg_integer(),
+             release_groups: [MusicBrainz.ReleaseGroupSearchResult.t()]
+           }}
+          | {:error, term()}
   def search_release_group(query, opts \\ []) do
     limit = Keyword.get(opts, :limit, 20)
     offset = Keyword.get(opts, :offset, 0)
@@ -12,26 +21,34 @@ defmodule MusicBrainz do
     )
   end
 
+  @spec get_release_group(String.t()) :: {:ok, map()} | {:error, term()}
   def get_release_group(musicbrainz_id) do
     API.get_release_group(musicbrainz_id, music_brainz_config())
   end
 
+  @spec get_releases(String.t(), keyword()) :: {:ok, map()} | {:error, term()}
   def get_releases(musicbrainz_id, opts) do
     API.get_releases(musicbrainz_id, opts, music_brainz_config())
   end
 
+  @spec get_release(String.t()) :: {:ok, map()} | {:error, term()}
   def get_release(musicbrainz_id) do
     API.get_release(musicbrainz_id, music_brainz_config())
   end
 
+  @spec search_release_by_barcode(String.t()) ::
+          {:ok, [MusicBrainz.ReleaseSearchResult.t()]} | {:error, term()}
   def search_release_by_barcode(barcode) do
     API.search_release_by_barcode(barcode, music_brainz_config())
   end
 
+  @spec get_cover_art({:musicbrainz_id, String.t()} | {:url, String.t()}) ::
+          {:ok, binary()} | {:error, term()}
   def get_cover_art(id_or_url) do
     API.get_cover_art(id_or_url, music_brainz_config())
   end
 
+  @spec get_artist(String.t()) :: {:ok, MusicBrainz.Artist.t()} | {:error, term()}
   def get_artist(musicbrainz_id) do
     API.get_artist(musicbrainz_id, music_brainz_config())
   end