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.ex

@@ -1,6 +1,7 @@
 defmodule MusicBrainz.ReleaseGroup do
   alias MusicBrainz.ReleaseGroupSearchResult
 
+  @spec included_release_groups(map()) :: [ReleaseGroupSearchResult.t()]
   def included_release_groups(release_group) do
     release_group
     |> get_release_groups()
@@ -9,27 +10,32 @@ defmodule MusicBrainz.ReleaseGroup do
     end)
   end
 
+  @spec releases(map()) :: [map()]
   def releases(release_group) do
     release_group
     |> Map.get("releases", [])
   end
 
+  @spec release_ids(map()) :: [String.t()]
   def release_ids(release_group) do
     release_group
     |> Map.get("releases", [])
     |> Enum.map(fn release -> release["id"] end)
   end
 
+  @spec included_release_group_ids(map()) :: [String.t()]
   def included_release_group_ids(release_group) do
     release_group
     |> included_release_groups()
     |> Enum.map(fn rg -> rg.id end)
   end
 
+  @spec url(String.t()) :: String.t()
   def url(id) do
     "https://musicbrainz.org/release-group/#{id}"
   end
 
+  @spec parse_type(String.t() | nil) :: :album | :ep | :live | :compilation | :single | :other
   def parse_type("Album"), do: :album
   def parse_type("EP"), do: :ep
   def parse_type("Live"), do: :live