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/secrets.ex

@@ -2,16 +2,19 @@ defmodule MusicLibrary.Secrets do
   alias MusicLibrary.Repo
   alias MusicLibrary.Secrets.Secret
 
+  @spec store(String.t(), String.t()) :: {:ok, Secret.t()} | {:error, Ecto.Changeset.t()}
   def store(name, value) do
     %Secret{}
     |> Secret.changeset(%{name: name, value: value})
     |> Repo.insert(on_conflict: :replace_all)
   end
 
+  @spec get!(String.t()) :: Secret.t()
   def get!(name) do
     Repo.get!(Secret, name)
   end
 
+  @spec get(String.t()) :: Secret.t() | nil
   def get(name) do
     Repo.get(Secret, name)
   end