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/release_group_search_result.ex

@@ -4,6 +4,15 @@ defmodule MusicBrainz.ReleaseGroupSearchResult do
   @enforce_keys [:id, :type, :title, :artists, :release_date]
   defstruct [:id, :type, :title, :artists, :release_date]
 
+  @type t :: %__MODULE__{
+          id: String.t(),
+          type: :album | :ep | :live | :compilation | :single | :other,
+          title: String.t(),
+          artists: String.t(),
+          release_date: String.t() | nil
+        }
+
+  @spec from_api_response(map()) :: t()
   def from_api_response(rg) do
     %__MODULE__{
       id: rg["id"],
@@ -14,6 +23,7 @@ defmodule MusicBrainz.ReleaseGroupSearchResult do
     }
   end
 
+  @spec thumb_url(t()) :: String.t()
   def thumb_url(rgr) do
     "https://coverartarchive.org/release-group/#{rgr.id}/front-250"
   end