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/artist.ex

@@ -2,6 +2,16 @@ defmodule MusicBrainz.Artist do
   @enforce_keys [:id, :name, :sort_name]
   defstruct [:id, :name, :sort_name, :country, :relations, :musicbrainz_data]
 
+  @type t :: %__MODULE__{
+          id: String.t(),
+          name: String.t(),
+          sort_name: String.t(),
+          country: String.t() | nil,
+          relations: [map()] | nil,
+          musicbrainz_data: map() | nil
+        }
+
+  @spec from_api_response(map()) :: t()
   def from_api_response(r) do
     %__MODULE__{
       id: r["id"],
@@ -15,6 +25,7 @@ defmodule MusicBrainz.Artist do
 
   # MASSIVE ASSUMPTION: if there's more than one Discogs link,
   # take the one with the lowest ID, as it's likely to be the main one.
+  @spec get_discogs_id(t()) :: integer() | nil
   def get_discogs_id(r) do
     candidates =
       r.relations
@@ -32,6 +43,7 @@ defmodule MusicBrainz.Artist do
     end
   end
 
+  @spec get_wikidata_id(t()) :: String.t() | nil
   def get_wikidata_id(r) do
     Enum.find_value(r.relations, fn relation ->
       if relation.type == "wikidata" do
@@ -40,6 +52,7 @@ defmodule MusicBrainz.Artist do
     end)
   end
 
+  @spec url(String.t()) :: String.t()
   def url(id) do
     "https://musicbrainz.org/artist/#{id}"
   end