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_library/notes.ex

@@ -2,22 +2,26 @@ defmodule MusicLibrary.Notes do
   alias MusicLibrary.Notes.Note
   alias MusicLibrary.Repo
 
+  @spec get_note(atom(), String.t()) :: Note.t() | nil
   def get_note(entity, musicbrainz_id) do
     Repo.get_by(Note, entity: entity, musicbrainz_id: musicbrainz_id)
   end
 
+  @spec create_note(Note.t(), map()) :: {:ok, Note.t()} | {:error, Ecto.Changeset.t()}
   def create_note(note, attrs) do
     note
     |> Note.changeset(attrs)
     |> Repo.insert()
   end
 
+  @spec update_note(Note.t(), map()) :: {:ok, Note.t()} | {:error, Ecto.Changeset.t()}
   def update_note(note, attrs) do
     note
     |> Note.changeset(attrs)
     |> Repo.update()
   end
 
+  @spec change_note(Note.t(), map()) :: Ecto.Changeset.t()
   def change_note(note, attrs) do
     Note.changeset(note, attrs)
   end