Documenting defstruct fields

By way of example:

defstruct [
  :name
]

@typedoc """
Yay docs
"""
@type t :: %__MODULE__{
  name: String.t | nil,
  age: age,
}

@typedoc """
You can create named types if you need to comment on the type of a field.
"""
@type age :: pos_integer | nil