Proposal: generator with spec

[Kensei Nakada]

Hi!

Phoenix generators create awesome files, but I want spec on functions generated from schema_access.ex as the following.

Thank you.

```

  @doc """

  Gets a single user.

  Raises `Ecto.NoResultsError` if the User does not exist.

  ## Examples

      iex> get_user!(123)

      %User{}

      iex> get_user!(456)

      ** (Ecto.NoResultsError)

  """

  @spec get_user!(id :: String.t()) :: %User{} | Ecto.NoResultsError

  def get_user!(id), do: Repo.get!(User, id)

```