Add more docs about :optional in Inspect

[sabiwara]

It wasn't obvious that we needed to pass a list of fields, I initially tried optional: true.
This makes it more explicit and adds a working example.

I thought it might actually be beneficial to allow some sugar like optional: :all for convenience, WDYT?
Otherwise I find myself doing things like

  @fields [foo: 1, ...]
  @derive {Inspect, optional: Keyword.keys(@fields)}
  defstruct @fields

[josevalim]

optional: :all is also fine by me!